I did find this on a vet website that sounds exactly what was happening:
"Another symptom that may fool you as to what is going on is to see diarrhea associated with straining in the litter box. How can a cat be constipated if it has diarrhea? The hard, dry stool sits in the colon and causes irritation resulting in fluid being produced. This fluid mixes with the surface of the stool, softening it and allowing it to pass past the constipation."
In my case it was just leaking out.
That is exactly what I was going to say...
I feel you should go for the ultrasound.
Also ask about the constipation.. that should be seen on an ultrasound and x-ray...
Good Luck...