My Life is About to Fall Apart

This all started in July. I've been extremely ill off and on and the doctors still do not know what is wrong with me. Every time I miss work I get a doctors note though, I eventually got FMLA because of how much work I was missing. This is unpaid which means I am barley making ends meet as it is. But now work is demanding I sign a release form for my medical history or get terminated. Which I am not even sure if that's legal. Ever since I got on FMLA they've been trying everything to make my approved sick leave unapproved so they can terminate me. Which is nonsense but my major thing is I cannot take the stress anymore. Anytime I call in sick HR calls and harasses me wanting all the details. My illness revolves around cycles of vomiting, diarrhea, fevers, and crippling abdominal pain. I do not think the stress is helping. I think its making it worse. Plus I don't even like the job. Management makes everyone's life hell and you are nothing more to them then a number and they make damn sure you know it. I absolutely hate that place even thinking about it makes me nauseas. I'm down to $800 and I fear on Monday they are going to fire me since I am refusing them access to my personal information. My question is do I put in my two weeks before hand? That way I left on my terms, and I get another two weeks of pay. They cant fire you if you quit right? My biggest fear is I do not have a for sure job lined up. I have applied at over 30 places and have started hearing back but nothing super solid or for sure yet. If I end up out of a job and completely broke I know my dad would step in and help before I become homeless and starving on the streets. But still I don't want to screw myself over. Basically I am between a rock and a hard place. Even if work doesn't fire me I can't stomach more than another month max. This place is just destroying my happiness. After work all I want to do I sit on the couch and cry. I am so stressed. On top of everything I am also a college student who is falling behind because I am sick every other freaking day. I think I could work with my illness in most environments but at my current job every time I leave my desk they time me. Which means when I come back from the bathroom I get my ass chewed out every time. They don't want me to come in if I have to spend time in the bathroom. I am starting to get better. I haven't thrown up in a couple days so I really think if I start a new job I wouldn't have to take sick leave. Which is one of my big fears that I will find a new job but get fired but missing work. I am sick and soooo freaking stressed and scared. I really hope everything works out for me, but so far things just keep getting worse.