What could this mark on my belly be?

I had this same thing happen to me about 10 years ago after laying out in the sun, mine was huge, looked like a birthmark purple in color, although I was scared to death, I had no insurance at the time to have it checked out, it took about a year for it to fade and go away, I have never had any problems since, although I often wonder and still worry some what about it. If I were you and you have insurance I would go and have it checked into, it may be nothing, but then again you never know, better to be safe then sorry, always better to catch things early on. good luck to you!