Thanks for your query.
A blunt injury
in the stomach might have caused a slight bleeding in the stomach/intestine. Or, the bleeding may have been due to a pain-killer, such as, NSAIDs, which you might have taken for punch-induced pain or for headache. The abdominal distension too may be due to transient shock of the intestine following the blunt injury.
Do NOT take pain-killer now. Instead, take proton Pump
Inhibitor such as Omeprazole
twice daily till your abdominal pain
Consult your doctor and apprise him of my opinion. I am certain that he agree with me; order required tests (if indicated), and- based on the reports- will prescribe suitable treatment as advised.
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Fond regards and have a nice day
Dr. Rakesh Karanwal