Sudden deterioration for ALZ patient. Any ideas why?

My MIL (moderate stage ALZ, 82 yrs) had a fall recently and hurt her ankle and was given prednisone for inflammation for 5 days which reduced the swelling. On top of that she was given erythromycin for cough for 5 days. She is highly diabetic and the doc had said that prednisone could increase blood sugar. Both medications finished a week back but since the fall she has been sleeping all day, not walking and not eating food. She was a very picky eater and now its even more that she is not eating her regular whatever she is used to have and is surviving on nutrition drinks. Its very hard for us to feed her and we are really exhausted. This has been going on for two weeks now. Do people deteriorate suddenly?