If you are quite skinny then it is pretty normal for this to happen, and be noticeable when you lie down and keep still. Basically your aorta (the main artery from your heart supplying your body) runs down the middle of your tummy, and because the pulse inside it is so strong, sometimes the 'shockwaves' from it can be visible on the skin in rather skinny people.
If you are worried about it, then go see your GP because it MAY be an Aortic Aneurysm (where your aorta becomes wider than it should be), but this is NOT very common in young healthy people- it is more common in older people with blood pressure problems etc. However i probably wouldn't worry.