For around nine months my 15 year old male indoor cat has had this thing where he strains, kicks his legs, howls and pushes his **** out; he even squats in random places like he's going to poop but doesn't.
After he poops in the cat box he is much more relieved and doesn't show signs of pain. He is not constipated, he poops all the time and it is soft, not diarrhea, but soft. (A mix of type 3 and 4 here: http://murbella.files.wordpress.com/2008/11/the-bristol-stool-form-scale.jpg
) He's had blood work, X-rays, ultra sounds, a probe, checked for megacolon & his anal glands checked. Everything was good. No masses, crystals, etc...
The vet said he'd like to try a high fiber diet before a colonoscopy because of the price but it hasn't been doing anything for him. My cat growls and grumbles when you squeeze on his lower abdomen so I believe it's something with an organ down there, I asked about renal failure and he said the ultrasound showed not.