Sneezing. Sneezing cause a tremendous force throughout you chest and abdomen. You can strain muscles by sneezing.
Wow. You must really sneeze hard! When you sneeze, it increases the pressure in your abdomen, some people have pressure in their bladder from it that makes them leak urine, but some just feel it in their muscles. It shouldn't hurt a lot, so maybe you should see your doctor to see if you have an abdominal hernia, or some other problem, which causes this.