So I am an 18 year old male, and I masturbate (surprise surprise) anyways, Today when I was finished i noticed that on the shaft of my penis a little lower than half way (so towards the scrotum) there was a red bump. I don't know if it is from friction or maybe a pimple or what. It is red, it doesn't hurt unless i pinch like popping a pimple but even then it doesn't really even hurt much or at all. It is on the right side of my penis. I masturbate with my right hand my hand makes contact with where the bump is. The part of the hand it touches is the palm right where the thumb starts to branch off the hand. There is only one bump and I have felt it and there is nothing hard underneath it, and there does not appear to be any puss on top. However I am scared because, 3 days ago my girlfriend of 3 months came over and we were naked and hugging each other and getting physical and are genitals touched but there was nothing more no insertion of any kind, and I just want to make its not an STD and just something from friction. It is on the top of the penis so her genitalia never, or should not have even touched there. Let me know if you need any more information.