I have throbbing/pulsing in my stomach too. Though it has only become really strong over the last few months. I'm now 19 weeks. It is the blood pumping through the body. If I put my finger on it I can link it back to my own heart beat. I only realised this after my doctor used the doppler on me and showed me the difference between my own heart beat which pulses down there and the baby's heart beat. Maybe it's that? It could be stretching too??