[livi]

Like any medical consultation the advice is based on probability. In your case, all facts considered, I would think its muscles in your lower ribcage area thats decided to contract sending pain nerve signals to your brain. Like Rick say, it could also be the double layered lung sack thats producing the pain from a small local inflammation - possibly infectious, most likely viral.

In any case, if it does not go away and definitely if it gets worse or you get breathing difficulties - go see a doc.

Disclaimer: Being a pediatrician the panorama of potential causes is somewhat different, although not much at your young age.