Hi
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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. This can be diagnosed and treated by a vascular specialist and an angiogram not ultrasound is done to detect it. Please consult a cardiologist to start with. You may need to consult a vascular specialist.
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. It is difficult to comment beyond this at this stage. Do consult your doctor. Take care!
Do you exercise? It may be you are pushing yourself too hard and have an accumulation of lactic acid, in that case I would trying stretching before working out. If not, then I would go to doctor and see if he can feel anything.
I have experienced this same problem. It usually happens when I am relaxed or trying to meditate. I have been told that it is just over used muscles contracting and trying to relax. We do not realize how much we use our abdominal muscles, but almost every move we make uses at least one muscle in the abdomen. If it is not causing pain, I would not worry about it. However, if it does become painful, you should probably speak to your physician about it.