i think it could just be your heart beat also. i have noticed that i can see my belly moving up and down slightly in time with my heart beat and that was middle of first trimester. from my understanding of things babys heartbeat wouldnt be able to penetrate all the fleshy bits between it and the outside of your belly. btw, i got one of the kmart heart beat machines too, much to realise i wont be able to use it for another 4-5 weeks!!!