How is Woody today, Emma?
I agree with everything Missy has said so far. If the vet suspects infection (cystitis) first, then the ideal treatment for this is Septrin, an anti-inflammatory plus plenty of fluids (water). A two week course of this schedule is the minimum I would recommend, and if no luck clearing it up after 2 weeks, then I would ask the vet for an x-ray to check for stones.
Treatment beyond that depends on what the x-ray shows. If no stones then a further two weeks of the same treatment; if stones they will need removing and post-op treatment can involve the same treatment as for infection, plus Cystease/Cystaid; if there is sludge/grit/silt/'crystals' then plenty of fluids is the most important, there are also additional treatments that can be used.