Hi!
In a thin built person, normal pulse of the abdominal aorta can be felt. This is a normal finding. Another possibility is a strong heartbeat being conducted to the stomach area. Again it could be a normal finding or it could be due to a heart condition. However it could also be an aneurysm of the abdominal aorta (AAA). Usually AAA and heart conditions present with other symptoms (increased heart rate, sinking sensation, sweating, etc) apart from a pulse that can be felt on the outside. This can be diagnosed and treated by a vascular specialist and an angiogram, ultrasound, CT scan or MRI is done to detect it. Severe acidity, gas in stomach and muscle twitches too could be causing a pulsing. If heart and aortic causes are ruled out, then this should be looked into. Do consult your doctor. Take care!
The medical advice given should not be considered a substitute for medical care provided by a doctor who can examine you. The advice may not be completely correct for you as the doctor cannot examine you and does not know your complete medical history. Hence this reply to your post should only be considered as a guiding line and you must consult your doctor at the earliest for your medical problem.
* an on and off pulsating feeling, I might add. It has been coming and going.