Prostate

I have had an overactive bladder for several years but for the most part it's been manageable. I am 42. Normally, I feel the urge to void about every 90 minutes. But it has started to interfere with important activities, like taking an intense admissions test for graduate school, and then I need one or two extra restroom breaks. The other time I feel the urge to go more often is after having an ejaculation. Then, I feel the urge to go about every 30 minutes all day long. I have finally sought treatment and my doctor gave me Vesicare which is supposed to be similar to Detrol LA but I can't see much improvement and I'm not sure if this is even what I need. I think it's a definite prostate problem; I'm just not sure what the exact problem is because it doesn't exactly fit any of the common problems that I've researched on the internet, like BPH. The ejaculation factor seems to be puzzling to my doctor and other experts I've consulted. One expert suggested cystitis or prostatitis, but neither of those exactly fits my condition either since I'm not in any pain. Any ideas?