I have a long haired female cat who is 13 years old. She has maintained a pretty steady diet of dry food and the occasional treat for about four or five years. Recently I added a wet food that had her favorite flavors instead of the dry treats. Around once a week for three weeks I would give her half a serving size (no bigger than the size of a thumb). She very much enjoys it. For a little over a week, she has been vomiting a mass of hair and bits of food, surrounded by a little amount of yellow liquid every other day or two. But, immediately after throwing up she returns to her cuddly, happy self and has maintained a thick, healthy winter coat. I have read this could be caused by the change of diet, but I was wondering if there were any recommendations for some at-home help I could give her to assure she is safe, considering her age she does not like to travel or meet new people.
Already I have tried
-Letting her out to get fresh air
-Brushing her with a longer comb to get any loose hair out
-Down sizing her food portions to avoid overeating