Cystitis is miserable for both cats and humans, and the frequent need to urinate is very uncomfortable. Fortunately, antibiotics will take care of it after a few days and the vet may prescribe another medication to calm the bladder and make her more comfortable. You may find that the problem will come back occasionally but that it will will stop completely as she gets older, that's what happened with a few of mine. It's good that you noticed it early on and that you are acting so quickly.