Hi, I understand your concern. From your medical history, I can see that you have undergone to intense physical activity recently (sit ups at your weight). Your muscles have undergone to this stress just recently, and got inflammed (which you feel as pain). It will take a few days until they get used to physical activity so no more pain. I advise to start slowly then gradually increase number of sit ups. Take oral anti-inflammatory meds you find over the counter (paracetamol or ibuprofen) and everything will be ok. No need to get checked for this time unless you have other unusual symptoms. Wish fast recovery.