She could "just" be experiencing gas pain. Has she been having bowel movements since the surgery?
Other more concerning cause of severe pain is peritonitis. This
could have been caused by the appendix if it were ruptured prior to
surgery, or it could be caused by an infection related to poor sterile
technique or a leakage of bowel contents when the inflamed appendix was
removed.
If you think she's been having regular bowel movements and if she has any fever, I would take her to the ED tonight.
Typical peritonitis pain is intense pain, fever, vomiting, weakness,
rigid abdomen very sensitive to touch. If it's peritonitis, it
can be fatal without timely antibiotic treatment.
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