Tough to say without seeing the lesion. I would be checked for various diseases and STDs - which can be done via simple urine study.
Penile vein thrombosis can be diagnosed clinically by a urologist or your personal physician. You can also consider an ultrasound of the penis.
If this is the case, you can consider non-steroidal anti-inflammatories, which should resolve the lesion within a few weeks.
I would discuss these options 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.
Medical Weblog:
kevinmd_b