Can doing sit ups / planks etc aggravate my gastritis worse ? as feels worse afterwards or can it be me stressing

Stomach pain: There should be no relationship of you strengthening the muscles of the abdomen that would affect the internal organs such as the bowels. This may be related to stress, but if you are adding protein shakes or other supplements to your regimen, this may cause some gastritis. Be sure you're getting plenty of sleep and continue workouts. If you still have problems, check which your care provider.