Sewathon? Sounds kind of like a bad case of sore muscles to me. Did it get worse and stiffer on the second day? That would be typical. :)
I'd give it a couple of days more before I go see a doc. Rest, gentle massage, gentle warmth and very gentle exercise (wriggling tenderly) help the muscles cope if that's all it is.
And just a suggestion for the future, I've see somewhere online it was recommended a phone book under the heel, to ease the muscle strain when sewing/quilting for long stretches. I do that now and find it much easier on my foot and shin muscles.