Prostatitis can be difficult to diagnose. However, with a normal ultrasound and rectal exam - it would make this possibility less likely.
You can consider a prostatic massage, and send off any prostatic fluid off to culture to ensure there isn't any prostate infection.
Another consideration would be a scrotal infection or condition. I would ultrasound the scrotum to ensure this isn't the case.
A cystoscopy can be considered to evaluate for any lower GI problem - such as a urethral stricture or polyp.
These options can be discussed with your personal physician.
Followup with your personal physician is essential.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Kevin, M.D.
kevinmd_b