Well it could be stress, as I know that this does mess up your stomach somewhat, and worrying about when you will recover is likely to be making things worse.
I suffered from food poisoning last summer, after a holiday in Tanzania, and that lasted about 5 weeks in total, so I know how you feel. I felt so ill, and was hallucinating and having deluded thoughts, as I was so dehyrated and unable to eat anything at all. But then it got to the stage where I was so fed up of not being able to eat, as I did feel hungry, and I knew that whatever I ate would make me ill anyway, so I started eating 'bad' foods, like chocolate/ crisps etc. I did go to the doctors but they made me do a sample, and I never gave it in, so I don't actually know what I had, but it did clear up by itself.
It does sound like you might have food poisoning, in which case the doctor should find something, or it should clear up by itself. Flat coke is good, as are the sachets you can buy to rehydrate yourself, although they taste rank!
I wouldn't go near your flatmates food, she sounds like a liability! Is there anyone else that is ill though? As it would be unusual if you were ill because of her hygiene, and she wasn't??