My son fell yesterday on his back, he fell on hardwood floor not from big height and there doesn't seem to be any external brusing. He is playing and laughing with us but every time we pick him up he cries like it seems to be sore under his arms but he fell on his back so I don't know what to do . Should hi have him x-rayd? Could it be his rib cage or simply soreness arm muscles? He is 18months old