What do you think is wrong with me? (need a diagnosis, please!)?

I'm a sixteen year old girl and I've been suffering from frequent urination for awhile now but over the past few months it's become increasingly worse. I drink only a glass of milk before school (in terms of liquids) and I have a hard time holding it for more than an hour and a half. I can't excercise because it makes me thirsty and when I drink 20 oz. from my water bottle, I go and pee like ten times in a row (like every five minutes). I've been tested for diabetes, a UTI, and all sorts of allergies and things (I also suffer from stomach pain) and they've all come back negative. Please give me any ideas for a diagnosis. I'm seeing a urologist soon but it'd be nice to hear what you guys have to say. This is a really embarrassing thing that I have to deal with so I'm willing to look everywhere for an answer. Thank you.