There is a FABULOUS new injectable antibiotic called Convenia (for both dogs and cats) that is extremely effective on UTIs. And it has no GI effects. One shot works for about two weeks. Expensive but absolutely worth it in my view.
My cat had a severe UTI/kidney infection, and stopped eating on Clavamox. It took about six weeks of Convenia to clear things up, but it worked, and no side effects. My cat's vet raves about what good results they're having with it.
Convenia is quite new, but you should absolutely ask your vet about it.