Twitching away right where you touch them is pretty normal. Sometimes mine do it, sometimes they don't--maybe the temperature of my fingers has something to do with it? Or maybe they just don't feel like being handled some days. Sometimes I'm a meanie and I "tickle" them, so I can watch the twitch move up and down their bellies.

But you said "spasm", which sounds more severe. And you said you have several bps, and you've never seen this before... I don't suppose you could take a quick video of it?