My grandma is 85 with dementia and for the last week every time she finishes eating she moans like if something hurts what can it be?

Needs evaluation: Unfortunately sometimes with dementia, there is a loss of ability to explain or communicate well. This is often due to loss of language functioning. Thus the groans and moans signal something wrong. Medical examination will be needed. Perhaps as simple as indigestion or possibly constipation or some other GI or bowel problems.