Since you are not sexually active and the sore appeared after masturbation, so it looks like friction blister. You can apply some calamine lotion as it will help in soothing the skin.Also wear cotton undergarments.Application of mild corticosteroid cream and oral antiinflammatories like ibuprofen/acetaminophen are also helpful. Also take care not to pop the sore or irritate it further. If the symptoms are severe then it will be best to consult a dermatologist.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
Hey - I got that same type spot in the same general area. I thought it was a friction burn too, did not hurt at all, reddish spot like yours. it happened when (had anibble or two of viagra) my wife pinched the head of my penis (hard - u can still see the marks from her nails - like a bruise that will not go away) then the spots as described started to appear. but i think the spot was a pimple from using an oily skin cream. the spot appears very red when wet and in the shower.......