What to do if i have a sharp pain inder my left rib cage?

Try stretching: To see if this helps or hurts. Raising your arms over head while back is arched will open spaces and if it is a muscle injury, will reproduce pain. If not, may be intercostal neuritis. If it is, there are injections that can calm this down. Either way, if it continues, consider seeing a physician for treatment plan. Hope that helps!