My cat twitches badly when he's laying straight. He seems happy and is purring but then his back legs will curl up and his back and bum will twitch. Then he'll relax and it will happen again. It may not happen for a few nights but has been happening on an off now for a few months. I think twitching may be the wrong word it's even more then that like a long twitch where his budy curls up a bit. Any ideas what it could be?