How fast did you change the food? If it wasn't gradual, the result is often diarrhea. Switching 20-25% per day or two can be more gentle on the GI tract.
Also, batches of food can be different in composition which sometimes has the same effect as fast food changes.
There are a number of conditions which include loose stools as part of the condition.
And of course, sometimes, cats can just get GI upsets from a cold.
You may need a vet visit to rule out any medical conditions.
Meanwhile, add a small amount of plain pumpkin or squash, with nothing else but water added (no sugar, etc) or 1/4 teaspoon of plain psyllium powder to soak up the excess fluid and give the feces some form.