The past week I noticed on an erection there is bump on my left shaft muscle of my penis. It is painless and hard. Seems to slide back n forth when touched. This lump can't be felt in flaccid state. Couldn't find the lump without erection. Had unprotected sex two months ago. The bump feels it's about 1-2mm in size. What could be the possible issue? What should I watch out for. Who is the best specialist for this problem?