Why there are times specially when lying down it seems that my tummy is also beating? is it connected to my heartbeat?

Palpitations: No direct connection between stomach and heart. However, if lie down particularly if the stomach is full and you are overweight, that can push on the diaphragm cause some uncomfortable sensation including palpitations at times. Try to avoid eating heavy meals at bed times as this could be the beginning of reflux symptoms. As well, avoid caffeine.

Yes and It's Normal: As our great father of medicine, William Osler, said, "In the normal abdomen one sees only the movement of respiration and the throbbing of the abdominal aorta." It's a really big tube moving a lot of fluid, so it's easy to see, even for first year medical students.