It's hard to say. I had a lyretail dalmation molly which started to "cramp up" into an S shape. I did a good water change or two, and was ready to move him into my new q-tank when he started staightening out.
He now appears to be fine. I'd put him into a q-tank (if you have one), do some regular water changes and watch closely for other appearing symptoms which may give more insight.
If you do not have a quarrantine tank, you may want to consider euthanizing him soon if he doesn't improve. You wouldn't want to risk losing more fish if he has something contagious.