Ican feel my abdominal aorta always pulsing sometimes its strong sometimes its weaker than other times only when lying on my back 19 6'0 163 lbs. aaa?

Good: That is likely your normal pulse in a relatively thin body! we all should be able to feel that-except most are too fat! that pulse is good. No pulsation would be a bad thing! your doc can assess for any expansion on physical or ultrasound.

Probably not: Very thin patients can often feel their aorta and become concerned that it represents an aneurysm. I wouldn't worry at all. And at 19 an aortic aneurysm would be really unlikely unless you had a connective tissue disorder like marfan's or ehlers-danlos syndromes, but this is likely something you'd have found out by now.