Mysterious Bladder Problem

I'm a healthy, 22year old female, and I've been having an array of problems for the past 7 months or so. It started when I began noticing a lot of iching and burning down there, when I went to the washroom and during and after sex. That went away on it's own, and I was then left with this constant need to go to the washroom. That subsided to how it is now, where I feel normal most of the time, but after I go to the washroom, I feel as if I need to go again... and again, and again, and it remains uncomfortable for about 30 min afterwards. There is no pain, no blood, and other than after I urinate, I don't feel the need to go more than I ever have. I've been tested for UTI, for stds and really, I'm at a bit of a loss. Anyone who has any ideas, please let me know. thanks.