What Causes Swollen Penis?

I masturbated for about an hour and a half straight earlier today (though it wasn't necessarily vigorous. It was just a constant rubbing). After finishing and cleaning up, I went to do some other things and now, a few hours later, I happen to check my penis and it's completely swollen up (I am uncircumcised). When I touch it, it feels somewhat numb (possibly from the rigorous friction). This has NEVER happened before. Help?