What is causing my abdominal pain and nausea? It's been MONTHS!?

I have been getting lower middle abdominal pain and nausea (but NO vomiting) for the last 2 and 1/2 months. It comes on and off. (No, it is not because of my period.)

I'm 14, female, 5'8, 122 lbs, started my period 2 yrs ago, NEVER have been sexually active.

What could this be?
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