Constant Urge to Urinate, strong smelling urine

I'm 26 years old and have never had a kidney or bladder infection until 2 years ago.  
The past two years, about once a month, I get the constant urge to urinate and it lasts about 3 days.  The problem is.  Nothing comes out and the only relief seems to sit upright on the toilet as if I was going to urinate.  A few days before the urge starts I notice that when I urinate my urine has a very very strong smell.  I really can't explain it.  It's not an Urine smell.. It's a little like ammonia. It is EXTREMELY strong.

To relieve the pain I take an over the counter urinary pain relief medicine and also have been prescribed Bactrim for it repeatedly.  I have been tested for STDs and everything is clean there.  

Any ideas of what the smell is caused from and why this always happens about once a month? Could we be looking at something more serious and some different testing?

[Lennox Hoyte, M.D.]

PLease have your urine cultured for a bladder infection. If this was negative, make sure your STD check included testing of the urethra. Also, your gynecologist should do a pelvic exam to evaluate for any abnormalities. You will also need to have a urinalysis. THe ammonia smell is not normal, and needs to be evaluated.

[pandora279]

A related discussion, same symptoms was started.