Sometimes when doing yoga, or having other physical activity i'll lay on my back after and briefly feel my heart beat in my stomach, is it normal?

This is normal: It's normal to feel your heart beating at times, particularly when you are quiet or still after an interval of heavy exertion. At those times, you may be aware of that feeling in your chest, abdomen, or even in your fingertips. As long as there is no pain involved, and the sensation fades as your heart rate and blood pressure recover with resting, you're OK.