I am not pregnant, but have all the signs...what could it be?

First, I am a 22yr old healthy female. Approximately 110 -115 pounds and 5'5". For the past month I'm constantly feeling hungry and in the worst cases I just feel miserable to where I feel very weak and sick and only food is on my mind. I get easily agitated during those moments as well. Sometimes when I eat I start to feel sick. I'll eat breakfast and by lunch time I feel as if I haven't eaten in about two or three days (no exaggeration....its horrible). I also have a constant feeling of being bloated. My period is 7 weeks past due. I initially thought that I was pregnant but I took two pregnancy test (one on my own and one at my university's med center) and they turned out negative. I don't know if this is connected or not but sometimes when I walk I feel pain in my left hip area to the point where I have to stand still for it to go away.