From your other post stress sounds likely.
I have a stress head bladder cat, i feed him mainly wet food, offer a water fountain and give cystaid as a supplement.
I have feliway plug ins and lots of good quality cat nip mice/valerian soaked toys for him to 'relax' with.
Feliway is a synthetic version of cat facial pheromones, it can help them feel relaxed.
Its cheaper online and if you visit vet uk through a rescue link they earn commission on your sale
Cystitis is often caused by stress, bacterial infection accounts for only 2% so antibiotics rarely do anything, its more likely the painrelief jab they get at the same time