I think I may be diabetic

Hello, new to the forum so I'm not sure if this is in the right section, sorry if it isn't. Well first some background info on me; 18 year old Male, a little underweight and not very healthy. About a month ago I woke up in the night with extreme thirst, I drank some water and went back to sleep, it was still there when I woke up and persisted throughout the day, I was also really tired. I must have drank atleast 3-4 litres of fluid that day and was still thirsty. I got an appointment with the doctor the very next day and said I was concerned that I might be diabetic and he asked me a few questions and such and was gonna take my blood sugar level with one of those device things but didn't have one in the room :roll:, so he gave me an appointment for a few days later to get some regular blood tests done and the results came back fine and I didn't think anything of it since the thirst and stuff had dissapeared, but yesterday when I was lying in bed the thirst returned and it feels exactly like last time, doesn't go away when drinking or anything, and I feel tired. I shall be making another doctor's appointment first thing in the morning and asking to be tested again. So my question is; do you think this is diabetes? Or merely something else (although I can't really see it being anything else as when I google "excessive thirst" or "extreme thirst" and such the only things that come up is diabetic things.) If you need any more info feel free to ask, thanks

Edit: And also another thing that makes me think I have diabetes is that since the first time I got these symptoms, there hasn't been much sugar in my diet at all (been drinking sugar free squash and water), but yesterday I pretty much overloaded with alot of sugar for the first time in a while (drunk about 1-1.5l of Dr Pepper and stuff) and then the symptoms started later the same day, coincidence? I don't think so.

Well I went to the doctors and had to have a urine sample which came back negative and they checked my bp and oxygen levels and they were fine, I'm still not fully convinced though I'll get a blood sugar measure at lloyd's pharmacy sometime soon, I asked what else it could be if it wasn't diabetes and she just said it could be because you ate alot of salty food, which I didn't so after I get my blood sugar tested, if it comes back fine I guess it's back to the dr, thanks for the replys.