I am 20 years old, 5'6, weigh 170 pounds. i've been feeling a pulse in my belly, specially laying down, lower back pain and headache. what can it be?

Aorta: Usually people can feel their aorta pulsate in their abdomen. It's a huge artery in the back of the abdomen and can be felt when laying down. The low back pain and headache should not be related. If these are bothersome, see your family physician for evaluation. Good luck.

Aneurysm: Your symptoms are suggestive of an abdominal aortic aneurysm and if you were 60, i'd consider it a good bet. But it would be exceptionally rare, probably unheard of, at your age. More likely you're feeling a normal aortic pulsation (best felt when lying down!) and the back pain is unrelated. The headache is a separate issue in any event!

Several issues: At your height and weight lower back pain is not surprising. Reducing your weight will do the same for your back. Headaches can be due to multiple sources and would need far more info. The pulsatile sensation in your abdomen might best be addressed by a physician examining you. Each of these issues would benefit from a detailed history and exam. See someone and feel better soon.