Having painless throbbing below naval. started yesterday & is still happening off & on; most common when sitting. can be felt with hand. worried- aaa?

It is not uncommon: to be able to feel an aortic pulsation, especially if you are on the slender side. It is reassuring that it is painless. It would be VERY unlikely for a young woman such as yourself to have a problematic abdominal aortic aneurysm. But the question is, "Why did you just start noticing this now?" I would have your doctor examine you and if there are any questions, do an ultrasound. Good wishes:)

Aortic aneurysm: can be felt in the abdomen, but generally above or at navel level is most common (ascending aorta). If you truly feel a rhythmic throbbing, and you are relatively thin and young, then it may be normal pulsation of the aorta. You can go see an MD for auscultation exam, and if he/she hears a distinct bruit (abnormal rushing sound of blood through vessel) then you may need to get arterial Doppler.

Doubtful to be AAA: Very doubtful that at 29 years of age for you to have AAA, unless you carry gene mutations for Marfan's or Ehlers Danlos. If you a very thin and scaphoid abdomen sometimes you can feel a normal pulsatile aorta. Any primary physician can assist you with this.