my guess is tendonitis. My first suggestion is to see a doctor. barring that, I would heat it, then exercise the foot by attaching a weight (even cans in a grocery bag) and doing toe raises, then icing the foot afterwards. Combine this with a thorough stretching program, and you may get rid of it in a couple weeks if you stay off the bike. Otherwise it could get worse and keep you off for longer and need cortisone shots.

Again just a guess. I'd see a doc.