You can consider an evaluation for systemic diseases, like diabetes or thyroid dysfunction. These can be evaluated for by simple blood tests.
A negative doppler ultrasound would make anatomical abnormalities or blood flow problems less likely.
You can be evaluated for prostatic infection - which can lead to the symptoms you are having. This can include a transrectal ultrasound to image the prostate as well as urine testing.
If the testing continues to be negative, it would not be a bad idea to be evaluated by a neurologist to rule out nerve dysfunction.
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