Britney Spears Calls Out Sister Jamie Lynn amid Book Release: 'She Never Had to Work for Anything'

Britney Spears is speaking her mind about Jamie Lynn's upcoming memoir.

On Thursday, Britney called out her younger sister Jamie Lynn on social media, following a Wednesday interview on Good Morning America where she opened up about their relationship.

In the lengthy social media post, the "Toxic" singer, 40, shared with fans that she had gotten "really sick" on Wednesday night and had a high degree fever. She then wrote she called her security guard to get her an Aleve to "relieve the pain," but the security guard said "no." She later tweeted that her fiancé, Sam Asghari, "got my medicine."

The singer then wrote she looked at her phone and saw Jamie Lynn, 30, did an interview to promote her book. And even though she watched while under the weather, she says "it was actually kind of nice having a fever so high cause I had to surrender to not caring."

After acknowledging that she can be a "drama queen" when sick, Spears assured fans she was feeling "fine today" after feeling "chill bumps everywhere" the day prior.

Britney then opened up about the things that did bother her about the interview.

"The 2 things that did bother me that my sister said was how my behavior was out of control. She was never around me much 15 years ago at that time .... so why are they even talking about that unless she wants to sell a book at my expense??? REALLY??? Then where the lady mentioned why did she accuse you of doing remixes to her songs ... I know it may sound like a silly thing to most people but I wrote a lot of my songs and my sister was the baby. She never had to work for anything."

RELATED: Jamie Lynn Spears Fires Back at Britney, Says 'Accusatory' Posts Are 'Not the Truth' and Lead to Death Threats

"Everything was always given to her," she added.

She concluded the post by saying she hopes her sister's book "does well" and slammed her family, claiming they "pull me down and hurt me always so I am disgusted with them."

Speaking about her new memoir Things I Should Have Said, releasing on Jan. 14, Jamie Lynn told ABC News Nightline anchor Juju Chang that she had always been Britney's "biggest supporter."

She later added, "I love my sister," as she began to cry. "I've only ever loved and supported her and done what's right by her, and she knows that, so I don't know why we're in this position right now."

Image Group LA/getty Britney and Jamie Lynn Spears in 2017

In this week's issue of PEOPLE, Jamie Lynn also said she's "only ever tried to be helpful, so any notion that says the contrary is just completely ridiculous."

For Jamie Lynn, "it was really important for me to separate myself from my family to focus on the family I've created, which is me, my husband and my daughters. Setting those boundaries in place was really important," said the Sweet Magnolias actress, who is raising daughters Maddie, 13, and Ivey, 3, with husband Jamie Watson, 40. "I am not my family. I am my own person. I fought very hard and worked very hard to establish myself and to build the life that I have today."

Last week, Britney unfollowed Jamie Lynn on Instagram. Jamie Lynn is still following Britney as of Thursday.

Throughout the final months of Britney's conservatorship battle, which ended in November, rumors of a complicated relationship between the two sisters came to light when Britney called out Jamie Lynn on Instagram for performing her songs "to remixes" at an awards show.

"My so-called support system hurt me deeply !!!!," Britney wrote in a July post.

RELATED: Jamie Lynn Spears: I Had to 'Separate Myself from My Family' but I 'Love and Support' Britney

Following the post, Jamie Lynn responded on her own page by preaching about peace in the caption of a mirror selfie.

"May the peace of the Lord be with you, and your spirit✌🏻❤️," Jamie Lynn wrote on Instagram. She later deleted the words, leaving only the emojis, after Britney appeared to mock the initial caption in a subsequent post. "May the Lord wrap your mean ass up in joy today," Britney wrote on a video of herself dancing. Britney later deleted the line.

In the past, Britney has blamed her family, including her father Jamie, for conservatorship abuse. (Jamie, Jamie Lynn and their mother, Lynne, have all denied wrongdoing.)

Jamie Lynn appeared to offer her sister support after she gave an emotional speech during her court hearing.

"Since the day I was born, I've only loved adored and supported my sister. I mean this is my freaking big sister, before any of this bulls—. I don't care if she wants to run away to a rainforest and have [a] zillion babies in the middle of nowhere," she said in a video on her Instagram Story at the time. "This situation does not affect me either way, because I'm only her sister who's only concerned about her happiness."

RELATED: Britney Spears Unfollows Sister Jamie Lynn on Instagram — Days After Calling Out Family

Jamie Lynn then said she's "very proud" of Britney for speaking up and that she has always supported her and her happiness.

"Maybe I didn't support the way the public would like me to with a hashtag on a public platform, but I can assure you that I support my sister long before there was a hashtag, and I'll support her long after," she said, referring to #FreeBritney.

RELATED VIDEO: Jamie Lynn Spears 'Blindsided' After Charity Declines Donation from Book Sales: 'Very Upsetting' Says Source

The Zoey 101 star first announced her book in October, explaining that she started writing it shortly after her daughter Maddie's near-fatal ATV accident in 2017.

While Jamie Lynn also announced at the time that she planned to give a portion of the proceeds from her new book to mental health organization and nonprofit This Is My Brave, the nonprofit later declined her donation publicly after fans of her sister Britney criticized her as being complicit in her conservatorship ordeal.

After the nonprofit declined her donation, a source told PEOPLE that the situation was "very upsetting" for Jamie Lynn, who has long been associated with her sister in both her career and her personal life.

"Jamie Lynn has suffered abuses in her life as well and that's what people aren't understanding — she has her own things she's gone through," the source said. "She can't help that she's Britney's little sister. Her life is only spoken about in relation to her family even though she has also been dealing with mental health issues for the better half of her life. This just reinforces the entire trauma — that her story doesn't matter."

Things I Should Have Said is out Jan. 18. For more from Jamie Lynn Spears, pick up this week's issue of PEOPLE, on newsstands everywhere Friday.