Frequent urination, but doctor says I'm fine?

I've been having frequent urination for a while now. I went to the doctor for it, he did urine tests, prostate exam, and an STD test. He said everything was fine, but I'm still going a lot. Basically, it happens a lot in the evening, and I'll go in intervals of maybe an hour or so. It doesn't happen all the time, and I've noticed that for some reason when I drink Vitamin Water or Sobe Life Water I have to go A LOT, like I'll sit down and get right back up to go in ten minutes. I had an appendectomy last summer, and I heard that surgery could affect this somehow? I always feel like I have to go, so please, share your wisdom. I'm 17, and I'm not always thirsty or losing weight so I don't think it's diabetes. Sorry for the long post, I'm just not sure what info I need to give.