My Horse Has Diarrhea?

We recently been feeding alfalfa and gr hay however I had two different bales (one gr and one hay) I found out the same guy i bought from had it all mixed so i bought those bales. I didn't do it slowly because they were all used to gr alfalfa mix now my 19 yr old mare has diarriah could that cause it she didnt have it until today,,,Another question not about the food but if a horse is stressed can they get the runs we brought a horse here yesterday and he has loose stool but acts healthy would could that be?