Told I have a hiatalhernia if I use my stomach muscles situps or from leaning back or sitting up on couch, bending get severe cramping pain rt rib why?

Acid Reflux. A hiatal hernia disrupts your stomach sphincter's ability to completely close off, and so acid may reflux upward with increased abdominal pressures or with certain foods. Acid irritation of the stomach lining or esophagus may cause severe cramping pain, decreased appetite, nausea, vomiting, or even ulcers. Avoid trigger foods like caffeine, spicy foods, and tomato. Also consider zantac (ranitidine) or pepcid.
Hiatal hernia. Hiatal hernia should not be associated with your current symptoms . It is usually related to reflux symptoms. You may have an abdominal wall hernia? See your doctor.