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.

Britney's also railing on her security for not getting her some pain meds, comparing it to the way she feels she's been treated by her family

Bottom line ... she says her relatives just love to bring her down, and that's why she prefers cats and dogs. That's ruff.