Overactive bladder?


Around 3 weeks ago I started with a pressing feeling on my bladder and I kept needing to urinate. I took a sample to the doctors but no infection was found. Now, I still keep feeling like I need to go but it only seems to be when I'm walking around outside. It's so annoying! And when I go not much even comes out. I have had UTIs in the past. I had an ultrasound of my bladder a few months ago but again, nothing was found. What could be the cause? And what can I do?! I'm 21 btw.