My vet doesn't seem too concerned about this so I wanted to ask you
guys as a sort of unofficial second opinion.
 My cat Spam has back leg problems due to cerebellar hypoplasia.  He
also has kidney failure and cardiomyopathy.  He recently had bloodwork
and such and he's stable.  However, he was having diarrhea which caused
problems.
 We got the diarrhea under control and he's back to his normal self.  
However, occasionally he seems to get an upset stomach... he'll meow,
then vomit a little, then poop where he's sitting.  It always happens
right after he's used the catpan, usually having pooped a little bit
while doing so.  It happens 2-3 times a month.
 At first we thought he was constipated but that's been ruled out.  The
vet doesn't seem to know what causes the problem and he doesn't think
it's a big deal because there's no diarrhea and no other symptoms.  Any
thoughts?  It's worrying to me because when Spam meows I wonder if he's
in pain.

Stacia