My 12 year old daughter says she has a pain in her right middle rib cage area. The one rib seems to be swollen more so than the other side. She says it hurts to breathe in and hurts to lay down like something stabbing. She is healthy otherwise and no know medical conditions.
This is most likely a non-displaced rib fracture or a muscle pull.  These are not uncommon, but can be tricky to diagnose.  A chest X-ray is the first place to start after a good physical exam and a careful history.  Often times there's a history of an injury that seemed minor at the time, but was followed by pain.  This condition is usually more annoying than dangerous, as it can take a couple of months to resolve. Other conditions need to be ruled out though, so the doctor visit is necessary to fully clarify.