If its not a Urinary Tract Infection, what is it?

Ok, another question..I'm 13 and I've been having this urge to pee alot, even when I just went, nothing comes out much when I go anyway. So, I went to the doctor a couple of days ago, and they tested my urine and said I didn't a UTI, they also did a sonogram to see if my bladder was empty, which it was, when I felt like it wasn't..It doesn't hurt at all when I pee, and I took some AZO Standard, because I have the symtoms of a UTI, when I really don't have one..so what is it?