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Oesophagitis can contribute.
For digestion, the stomach demands more blood after eating / It has to be diverted from another area or the hear has to pump more. (the rate or the volume per beat is upped)
Reclining pushes the diaphragm up and may contribute
Gas is not produced in the stomach (in the intestine, yes) / it is usually the swallowed air / try eating with mouth closed...
The medicines are appropriate.
Wishing speedy recovery