My Daughter Has Just Turned 12. For The Last Year

My daughter has just turned 12. For the last year (approx), she has complained of her ribs on one side (left) popping in and out of place. It begun on a middle rib but over time has increased to the top and bottom ribs. When she feels it pop out, she can usually manoeuvre the rib back to normal, but occasionally it doesn't want to go back. Once it happens, it will happen more frequently over the next hour or so? Do I need to go and get her XRayed or checked out. I thought it may just be a temporary growing phase but it has gone on so long, we are starting to worry now.