It could be exactly what you think, Robyn... Cramps. Or he may have twisted it. It happens to my pups (or rather, my boys) fairly often. Arthur hurt his hip but with rest, it was fine. And Charlie seems to twist something at least 1 a week. But he's learned how to play the sympathy card. hehe

If you see Jasper limping again, I'd suggest maybe giving him a chewie and keeping him off his feet for a while if you can.