u need go to the doctor and have it check out
Does it hurt when you breath in? If so it could be inflammation in between the ribs like pleurasy. You would get checked by a doctor so they can listen for fluid in between the ribs. They can give you an antibiotic for it. A hot water bottle or heat pack will help. If it is sharp and goes away off and on maybe it is just gas. if this is the case try GasX and walk around and it will eventually go away. Let me know if the pain is different than this.
trapped gas, pulled muscle ,go to gp they should refer you to consultants at the hospital