Also, I felt my baby move for the first time at 15 weeks when I was laying on my back. she felt like a little lump moving to curl up on the right side of my stomach. So marissa could be right :)
I think right now you're fine, but eventually it's bad for both you and baby if you lay on your back because the weight of your womb will eventually press onto a large vein in your back. Your body could be trying to tell you thats not the best position for you anymore
It couls be baby fluttering.