pressure on the nerves that run between the rays of your feet. The next question is why was there more pressure on 'em today?
there are a gazillion things that could've done it.
shoes laced too tight/feet swelled. Fewer breaks. Pushing harder on the pedals Longer ride than usual. Irritated nerves from previous shoes or ride. (been wearing any high heels lately?)
On the structual side, things could be getting irritated enough that now they're starting to complain: dropped metatarsal head. over pronation. neuroma. bunion.
and so on ad infinitum!
I think if you search "hot foot" or "numb foot" Wahine had a really good thread. If you have more specifics, I'd be happy to try to help, too. But your best bet is to experiment a little, and then go see a doc or PT if things don't resolve.
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson