It does sound as though the stress of the op has sparked off another bout of cystitis.

I would ask your vet about Zylkene, its a natural product (just sprinkled on food) that can help cats relax more, use it along with the Feliway. Maybe also ask about Cystaid, it sooths the linining of the bladder and it may help if she's going to be prone to cystitis. Both products can be brought online much cheaper than your vets though.

Wet food is better, but should be high meat content and no cereals/grains/veggies, avoid dry. You might try a water fountain too, to encourage more drinking.