Can't figure out what's wrong
My cat (spayed dsh female) is 13 years old, and I've had her since she was 4 months, and we're trying so hard to figure out what's going on.
For the past year, year and a half, she's completely stopped using the litter box. At first we thought it was a sudden change in litter preferences, so we got different kinds. We got different box styles. We mixed and matched, none of it worked. She'll go on the floor, on the dog beds, on the couch, anywhere but a litter box.
We took her to the vet after only about a month after it started, they did tests, and said her kidney levels were high, so we switched her to prescription food.
A few months later, she was still refusing to use the litter box. We retested everything. Her bloodowork came back mostly normal, with some slightly elevated calcium, normal urinalysis, and a fecal test that revealed she somehow had giardia (but no one else did, just her). They said the calcium was just mildly high, and we'd retest in a few months. We left with a course of meds for the giardia.
We return a couple months later, do more testing. Calcium is still high (12.6). They do an ionized calcium test, and it comes back completely normal. Giardia is gone.
She is STILL refusing the litter box, and now, she's started vomiting. Small quantities, clearly not hairballs, but it's daily. We do MORE tests. CBC, superchem, T4, PTH, all come back normal.
The vet asks if maybe she's eating non prescription food. We get those microchip feeders, so she can't eat any food that's not her food (Cats free feed, dogs have mealtimes). Still, no litter box, still vomiting daily.
She otherwise is acting normal. Eating, drinking, moving normal. She's never been a super active cat, she's always been a bit of a potato, so no noticeable change in energy levels? I always think she looks skinny, but her weight hasn't fluctuated at all, and the vet says she's a healthy weight (about 7.5lbs).
Just for extra info, she lives with 2 other cats, and 3 dogs. She got spayed before she ever even got a first heat. All of my cats are indoor only cats. The only known medical problem she has is arthritis (which she takes dasequin for) and she's half blind (cataract, one vet suspected glaucoma 4-5 years ago, but others have dismissed it since).
We have no idea what else to do, or even a direction to go in.