if your cat is not going in the litter box, it is very likely that there is something physical going on. see if you notice the cat walking, standing or sleeping in any odd positions. even if you don't, it could still be physical. remember cats are very private creatures and will hide stuff from you (like if they are sick) my vet told me that we often put human emotions on cats, such as "he is angry, so he is going to the bathroom", which isn't really true. it's usually a dominance issue. you can be giving the cat plenty of love, but if it feels like the dog is in his space, or in his spot, or eating his food, THAT'S going to be what the real problem is. and contrary to popular belief, litter box avoidance is more common with cats who have covered litter boxes. it has something to do with the way they find their smell.

i swear im not a know it all; i spent $5000 on my cats last year because of UTIs, bladder stones, and litter box avoidance.
if you want to know anything else, im sure i could dig up mounds of information that my vet gave me. i hope it gets better soon!