I can feel a strong pulse below my stomach. Does this mean that I have an aaa?

Maybe. A very slender person will likely be able to feel their own aorta. Most people should not be able to do that. If you have any concerns or if you have family history of aneurysm disease then you should have an ultrasound. If you are a male smoker with a family history of AAA then you should definitely have an ultrasound.
Not necessarily . The physical exam is very bad at predicting an aaa. Ultrasounds and ct scans are better tests to screen for aaas. However, it is possible to feel the pulsatile AAA in a thin person- it is usually felt above the belly button.