I'm extremely thirsty. Of what could this be a symptom?

I’m a reasonably fit 16 year old girl (113 lbs, 5’2”).
Over the summer, I noticed I was drinking a lot of water. It lasted into the fall (I thought it might be related to my summer activities). So, out of curiosity, I measured my water intake on Thursday. I drank three gallons in that one day! I was expecting to drink about a gallon. But three… 0.o And this was regular for me!
So my mom and I asked some friends, and everybody said to get checked for diabetes. We got the test done today. They said I was perfectly normal; the thirstiness is just a fluke! They checked my blood sugar, liver and something else (can’t remember what).
If I restrict myself to a gallon or so in a day, I get an extremely dry, sore mouth and throat. I also get a headache, get all jittery, and get super hot, even though my hands are cold to the touch.
So… If it’s not diabetes, what is it?
I did some googling (even though I don’t put much stock in it), and I learned a bit about diabetes insipidus, which my mom thinks I have. I don’t think so. From what I read, DI has little connection between urination and water intake, right? Well, it’s directly connected for me. Drink a lot = pee a lot. Drink nothing = pee nothing.
So… I couldn’t really find any other information. What else could be causing this?! I’m tired of drinking so much water and peeing ALL THE TIME!! And I’m tired of the headaches, jitteriness, and pain in my mouth if I don’t drink the water!