What Causes Feeling Of Movement Inside Stomach?

I have been feeling some type of movement in my stomach for past 6-8weeks now. The movement is on my left side of my stomach. It will move around for a few seconds an stop,than as soon as a few minutes go by it will start again. I know i am not pregnant because my tubes are tied. My last menstrual period was last month an every other month before that month it was coming on. Whats the problem? I haven't seen a doctor in awhile an my last check-up was in 2007.