Hi. Thanks for your query. Noted your history of tightening or hardening of stomach muscles especially on crunching. It stays for few minutes and goes ways, a little movement and comes on. The commonest cause of such a situation is overwork of the abdominal muscles that might have caused by the exercises or strenuous work or so on. Another possible reason is problem with spinal cord or motor nerve roots compression. I would advise you to go for clinical evaluation, examination and tests as per the clinical findings. Take rest for a few day as muscular strains need rests to have relaxation as well as get the medicines for muscle relaxation.