Assuming it is really your stomach muscles that are hurting, the pain is likely caused by the separation of your rectus muscles/abdominal muscles (the formal medical term is DIASTASIS RECTI) that occurs as the pregnancy enters the third trimester. This occurs in order to make room for your ever-growing bump. Earlier in pregnancy, having sharp, shooting pains in the belly is often the result of the round ligaments stretching. Some women also experience shooting pain around their belly button from increased pressure from their enlarging baby and uterus. The "soreness" from the separation of your rectus muscles is normal. It is important, however, that any pain be discussed with your practitioner so that more serious things (like preterm contractions) be ruled out.