Hi, Thanks for contacting Healthcaremagic. I appreciate your efforts to keep good health even in seventies! I have gone through your problem and i am of the opinion that you are most probably having some kind of hernia. I will briefly explain the reason for the same.As you age the strength of your abdominal muscle gradually decreases and any increase in the abdominal pressure may cause protrusion of abdominal contents through weak portion of your belly. Sit ups and crunches significantly cause increased intra abdominal pressure and it may be the prime reason behind your problem. Visit a general surgeon for proper evaluation and treatment,you may require some minor surgery to correct the defect. You should consult a physical trainer before getting in to heavy gym training. I hope your doubts have been clarified. Thank you.