Small blister on the shaft of my penis, doesn t itch, doesn t hurt. There is only one small blister. I was masterbating pretty hard the other day, I m on antidepressants, so it takes a little more effort sometimes. Is masterbation the reason it is there. The last time I had sex was on August 7, 2015. So I m thinking that if it were herpes, it wouldn t be showing yet? Any help at all would be great.