Can feel pulse above my navel standing by pressing in, sometimes after exertion, sometimes not. 21 y/o, around 12.5 stone, non smoker could it be aaa?

Not likely: if you are thin, an aortic pulse is usually palpable in that area. However, while you say you weigh about 170 lbs, that is thin if you are 5'10 and fat if you are 5'5, so I don't know whether you are or not. What you seem to be, though, is excessively focused on having unusual conditions not typically associated with being 21. That may be your biggest issue. Talk to your doctor.