Hey dude, the EXACT same thing has happened with me :/
Ok dude i know this is kinda late but im the same age as you are and have the exact (EXACT) same thing as you described it, iv'e always wanted to know what is it but i don't. I'm pretty sure is nothing dangerous but just leetting you know you're not alone ;)
I had the same thing happen to me when i was probably about 15. It made me extremely self-conscious and more unconfident around girls than I already was. 6 years later my penis hasn't fallen off, I've never asked a doc about it, but I haven't had any problems and no girl I've slept with has cared. I wouldn't worry too much :)
Sorry, going to bump this again in hopes that somebody sees it.
So is it possible for a "birthmark" to appear in teenagehood like I have described? Should I be concerned? Some peace of mind would be incredibly wonderful.
Anybody??????????????????????