First of all, fish antibiotics aren't going to treat a rat. They're not in the proper dosages, and most vets will prescribe either doxy or baytril to a rat who has a URI.
In addition to that, it sounds like you should probably find a better home for the rat. The roommate had no reason to THROW the rat. That can kill a rat, and is going to scare her badly. She needs a single sex friend, and she needs a home that is understanding that she needs socialization work.
I've gotten rats that did not have very good socialization when I got them, sometimes even to the point of biting. You need to be patient and understand how to work with them. Flicking her when she bites is encouraging aggression.
With a sudden behavior shift, I'd want to check out to see if the rat is pregnant. Females get a bit testy later on in their pregnancies. But it could also be due to the illness, so you might want to have a vet check it out.
To get a rat to stop biting after it already fears humans, you need to go slow and earn the trust back. If she bites, Do not yell, do not scream, do not react. If she learns biting makes people go away, and she doesn't want to interact with people, biting seems like a good thing for her to do. (In her mind.) Offer her treats, and watch her body language. It'll take time and a lot of work to earn her trust back. If you see she's going to strike, back off.