You may be thin, or you often exercise your abdominal muscles. When you lie down, your abdominal muscles will collapse, so you will feel your abdomen is hard. The part that looks like a heartbeat is the abdominal aorta. You press it with one hand, and then feel the pulse with the other hand. You will find that its beating is consistent with the beating of the heart. This is when the heart beats, sending blood to the abdominal aorta, and the abdominal aorta also beats. This situation is normal, don't worry! If you are obese and have thick abdominal fat, this feeling is light or not.