I would see a doctor about this issue. It could just be muscles, but it could also be something to do with your digestive tract, which is often an easy fix.
I used to get intense muscle spasms in my stomach around my belly button area and was given dicyclomine (sp?) which is a pain medication that controls stomach spasms.
Hope you figure out what it is, it sounds uncomfortable.
It is quite possible that you are having gas cramps, which can be felt in the front and in the back too. You might try taking some Beano prior to eating any gassy foods and if it's too late to do that, some simethicone over the counter medication and see if it winds up helping.