4:49 PM PT -- She didn't mention it before, but for whatever reason, Britney just clarified her fiancé Sam Asghari helped her score some medicine, tweeting, "Pssss my fiancé got my medicine ... I survived !!!!"
Britney Spears says she's got a fever so bad, it feels like it could be the end ... and, yet, she's still taking shots at her sister Jamie Lynn over her book and recent interview.
Brit just posted a long rant, detailing her symptoms ... saying she had a 104-degree fever Wednesday night and complained her security wouldn't leave to get her Aleve. She admits she's being a drama queen by saying she's possibly dying, but says she never gets sick.
While Britney makes her illness sound very unpleasant -- drama queen or not -- she says it actually made it easier to stomach Jamie Lynn's interview with "Nightline" ... even though Britney's got some big issues with what was said.
Britney's got 2 bones to pick with Jamie Lynn ... saying it's ridiculous she said Britney was out of control 15 years ago. Britney says her sister was not around her enough to speak on the topic and chalks it up to Jamie Lynn trying to pump up book sales.
The other issue Britney has involves Jamie Lynn remixing some songs ... and you can read her full rant about everything.