It could be a number of things:
foreign body, such as a grass seed. may have even stood on something sharpe - making it painful and swollen.

the best thing to do is inspect the swollen foot, inbetween the toes and pad. move the hair around on the foot to inspect it further.

see the web link on how to treat a swollen paw: How To Treat a Swollen Paw | eHow.com

hope this helps!