Sorry, I'm not sure what the proper name for it is. It could be my prostate but I hope not. Anywho:
I'm 16 and I've had problems in the past that I went to my doctor for. It took him a while to find out what it was, but he finally decided that my "prostate valve" (whatever that is, Google doesn't bring anything up) was tightened up from taking ADD medication in the past so he prescribed Flomax, which seemed to work. It stopped hurting when I coughed deep, the burning while I urinated went away, and I started lasting longer during sex.
I am an abnormally big guy (16 years old, 6'5) so I don't know if that has anything to do with it.
Anyways, it's not a big enough concern to go to the doctor about but I wanted to get some advice. I've noticed recently that when I get an erection I feel it swell up.
This has never happened before and it doesn't hurt, but when I'm sitting down I feel the pressure. This happens when I just started to get aroused. My penis doesn't have to be like a rock but I don't have any problems getting erections (I used to, but I think it was just the phychological fear of doing bad in bed since I was still a virgin), so in my mind that rules out prostate problems but I'm not the doctor.
One other thing to add, I don't really have premature ejaculation and I know everyone wants to last longer but I can manage about 5 minutes. That's with moderate thrusting and the start/stop method. I think this is just due to poor diet with little exercise which I'm too lazy to change.
You mention that you have a poor diet and don't exercize as much as you'd like to... give that a go. Diet and exercize are pretty important. I agree with Georgia59, PE is normal when you're hormones are in cahoots (unfortunately it's a fact of being a teenager).
If the problem persists, though, go see the doctor again, or get a second opinion.