I would be examined by a urologist or dermatologist. It may be possible that the lesions are from masturbation (causing some sort of dermatitis). Herpes or other genital lesions (i.e. genital warts, chancroid, fungal infection or a cellulitis). You stated that the blood test for herpes was negative. Another consideration would be to take herpes cultures when the lesions are more prominant. To be complete, I would have my urine checked for infection, including tests for gonorrhea and chlamydia.
If the urology referral is not revealing, you may want to consider a referral to a dermatologist.
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.
Thanks,
Kevin, M.D.