If there is anyway at all that you can swing it, you should try to find a vet that will run a culture on that RI.
Up until you know what exactly you are up against, treating her with different meds are shots in the dark...esp. since this has now turning into a somewhat chronic problem.
She needs to be hit with the exact meds that will kill the organism that is making her sick, and the only way to figure that out for sure is through a culture. You could call around to different vets
to see who does this, how much they charge and if they possibly take payments...
Good luck, its scary when your loved ones (2 legs, 4 legs, no legs) are sick..