Colitis is a fancy word for inflammation of the large intestine. While the most obvious sign of colitis is diarrhea, sometimes it will hurt the cat to poop. Thus, in trying to hold it in, the cat may develop constipation.
There are many causes of colitis, including bacteria, fungi, viruses, allergies and parasites, among other diseases. Signs include straining to poop, lack of appetite, dehydration and vomiting. Your vet will test for the underlying cause and treat it accordingly. This may include a more fiber-rich diet, de-worming, antibiotics, laxatives and/or fluids.