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I read your query and understand your concern.
I do not think being hit by a doorknob has anything to do with diarrhea.
Do you have any associated pain or bruising in the affected area?
If you do not have recurrent loose stools, then nothing has to be done.
But if it recurs, you may have to take Lomotil (Diphenoxylate and Atropine) and antacids to control it.
Consult your specialist doctor, discuss with him or her and take the medicines with their consent.
As you said that you have a long flight, it is better to take some precautions.
Also, it is advisable to carry something for pain management.