hi, I understand your concern. The muscle movement you are referring at seems to be movement of gasses.- It is somewhat similar to fetal movements in pregnancy. In old age digestion is weak, intestinal movements are slower- making improperly digested food to lie in bowels for a longer time, leading to excessive gas production/ constipation. * You are 75, must have attained menopause-- so posseblity of pregnancy symptoms are as good as nil. * you will have to get abdominal USG scan / be examined by a gynecologist -to remove your doubt about tumor. * Mild regular exercise( walking), small multiple servings of food,increased fibers in diet with increased water intake / addition of mild laxative like Ispaghula along with use of digestive enzyme would solve your problem. Thanks.