Hernia , bruised rib or pulled muscle , can be caused by sleeping the wrong way, give it 3 days if its doesnt get better get it checked by doctor, dont think its anything serious, hope that helps
have you been coughing a lot. some times you can pull something in side and make you sore for a few days. even hurts to take a deep breath. I have had this problem. doesn't last very long. mandy876