I'm not sure what is causing it, but I have had something similar. I tend to get mussle spasams sometimes, and my stomach does it rarely. It usually feels like a twitch though and has never caused me worry. You may want to try webmd.com

That site is a goldmind when you are scared something is wrong. How long has it been going on? Is it painful? Most likely though it is something with the abs. Did you work out today?