Thanks everyone! Guess you're right about the dirt, but I guess the damage was already done in the hospital when I looked after my room-mate's kids three days after surgery; after that, I noticed the swelling for the first time.

I googled meanwhile and I'm pretty sure it's a seroma. Obviously they do occur when you're walking around too much after surgery. They're usually punctured but that would increase the risk of infection. I'm going to see my doctor again in an hour as the lump has gotten bigger during the night.

Kirsten