I'm not doctor mo, but something like that happened to me last year in that same area. I think it is a lymph vessel.
First time it was from wrapping [words=http://www.mattersofsize.com/forum/showthread.php?44-Ace-Strapped-Jims-Joint]UJS[/words] too tight, the second was from using the [words=https://officialhydromaxpump.com/?uid=6&oid=2&affid=98]Penomet[/words] my first time. I've also heard that this can happen if you have really rough sex for an extended period of time.
Just don't do Penis Enlargement, masturbation, or sex and it should go away in 1 or 2 days, maybe longer but I've only had this twice.
If it is painful, then it could be a t-vein.