Well, we don't have enough details to tell anything for sure..it depends what kind of injury is that.Most common muscle injuries are strains, sprains, cramps and repetitive stress injury..it can also be sthg with tendons (they connect muscles to bones)..so the list is wide. You can find info on it on many medical sites, here's a brief def. from one of them:
About sprains and strains
Sprains and strains are types of minor injuries to the soft tissues and connective tissues of the musculoskeletal system. Sprains usually refer to injuries to ligaments, but sometimes to other connective tissues, such as tendons and the capsules surrounding joints. Strains usually refer to injuries to muscles or to the areas where muscles become tendons.
Sprains and strains may occur together, and occasionally are quite severe, requiring immobilization of the body part in a rigid cast for weeks, long-term rehabilitation programs, and sometimes surgery.