What would be the symptoms of a stomach muscle injury?

Stomach or abdomen? If you mean muscles of the stomach, injury likely will result in both dysfunction (slow stomach emptying) and/or perforation (example: ulcer, ith associated burning). On the other hand, if your question pertains to the abdominal wall, then injury can include herniation, incarceration of an organ, muscle tear (with associated pain), puncture wound from a sharp instrument versus blunt trauma.