Diarrhea in Cat

One of my neutered male cats has been having problems with diarrhea. I have fed them Chicken Soup for the Cat Lover's Soul and never had any problems. About 6 weeks ago I forgot to order my cat food so I ended up getting a small bag of inexpensive food (I order my cat food on line because there are no stores for 20 miles to get good quality pet food) No problems. Then about 2 weeks after going back on their regular food one of the boys started having liquid stools. He goes to the box then has a projectile liquid fart just before he gets in the box. My vet checked his stool - all clear. I gave him medication for 5 days that tasted horrible but finally stopped the runs on the last day. He was good for about a week but started having random squirts. I am now wondering if he could have developed a food allergy even though he is eating the same food he has had for several years. He hasn't lost any weight and is normal otherwise.