What Causes Bruising In Penis After Sexual Intercourse?

This happened last night. I am a 33 year old male, never had any sexual issues.

I was having some pretty intense intercourse with my girlfriend, when all of a sudden my penis started ballooning in size. It didn't hurt, I just noticed it got very tight all of a sudden. I pulled out, and noticed that my penis was severely swollen, basically bruising up bigtime. No blood ever left my body, it was all inside the skin. Nothing was coming out of the tip. No signs of blood anywhere, except for a hugely ballooning penis shaft (which was still very erect, and gaining huge girth by the minute...).

We examined it, and found that sacs of liquid (blood, I'm assuming) were forming in the skin, all the way down to the start of the head (I am circumsized). (Nothing entered the head area, it remained normal size).

This morning the swelling has subsided. But the area is deeply bruised black/blue. There has been no pain. Its just ugly really. Anything to worry about?