What Causes A Hard Bump Under The Skin Of Penis During Erection?

HI, I have recently noticed when i have an erection that i have a small Hard / bee bee size hard thing under the skin of my penis. I can t tell what it is. It doesnt really hurt however it seems to get irritated a little after working things up in that area. Of course when things are not worked up, i have to search to find the hard spot, but can after looking for awhile. I am in fear it may be a blood clot or something like that. I have been married fro 15 years and don t have any sex with any other people and don t think is in any way an STD. Again, it is not painful, just freaks me out. doesn t seem to be an infection either... im 39 years old.. just noticed it about a week ago. please help. Feels like a bee bee from an air gun when fully erect.. and it is not on the surface, definatley a bit under the skin...