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I must say this is an unusual location for a 6 year old to have significant pain. The location suggests heart pain (very unusual in a 6 year old), lung pain (not sure what it would be from without a fever or trouble breathing), a stomach issue (ulcers in that age are rare but not impossible) or, as you noted, muscle pain.
The botXXXXX XXXXXne is that ANY "severe pain" of unknown cause needs to be checked immediately by an MD.
Feel free to ask a follow-up question.
A colon obstruction would manifest as constipation, more general abdominal pain, and vomiting. Constipation itself would generally be lower abdominal pain.
A flu would not usually give localized pain; it usually gives generalized body aches (and definitely fever).
And a BONUS is always appreciated.