Causes of frequent urination?

I have been having to urinate alot lately, most of the times its at night i get waken up with the urge to pee like 3 times, during the day its not so bad, also sometimes I feel the urge to pee and like barely anything comes out, the other night though I must have had to go about 10 times minimum, and even after i still had this feeling like I needed to urinate, then like days like today I have felt normal.. any idea? I went to doctor 2 days ago and I dont have a UTI.. its been going for about 3 weeks but its been at its worst the last couple days after the doctor, im starting to think about it so much i think thats part of the problem but im not sure, no its doesnt burn when i pee, im 23 years old, ive had lower back aches but nothing painful..