Interesting. I don't think that's a bruise. Is your foot changing colour? If its not at all painful and not changing colour I doubt its a bruise. Sounds more like a bite or infection rather then a muscular injury or a fracture. You'd really feel either of those.
I was bitten by a spider on my hand and my whole hand swelled up like it was going to burst out of its skin but I couldn't really feel anything. I also got stung by a bee on my toe when I was drunk and walking around bare foot. A day later it swelled up like crazy without pain and I didn't know why, cause I couldn't remember getting stung
and it stayed like that for a few days. Just ice it dude, and maybe take some anti inflammatories.