Urinary retention at 25...what the heck is going on??

Hi,
I've had on and off issues with urinary retention for the last year or so. Seems to come and go, sometimes only lasting a day or two and other up to a couple of months. It just started in again last week following having a bowel movement.
Basically, I'm able to urinate but always feel like I never fully emptied.
I've been to my doctor and urologist I don't know how many times. I've been on antibiotics a couple of times in case it was an infection. I'm currently on Uroxatral (Alpha-Blocker) which seems to help a bit.
I also had a cystoscopy done and nothing was found.
My urologist doesn't believe there is anything majorly wrong with me. He thinks my issues may be due to stress and the tightening of the bladder muscles because of it.

It definitely seems to get worse just following having a bowel movement. The rest of the time it is for the most part, manageable.
I did have a bladder scan done and the post void urine volume was higher than it should be (97ml). But they did it almost a half hour after I had urinated and I already had to go again when the scan was done, so I don't know how accurate that is in my case.

I work in law enforcement and much of the time we are sitting down in a cruiser, could sitting so much be part of it?

Has anyone had similar issues? Why would it get worse following having a bowel movement?
Just troubling...I'm only 25. I shouldn't have issues like this...
Thanks in advance! :-)