If it was dexamethasone that would be about right for it to wear off. There is another one called depomedrol which lasts about 3 weeks. Among the steroids I think cats do best with prednisolone, it is a daily oral medication. It isn't normally used for this, it is the steroid of choice for IBD. I think the improvement and the decline means it is some type of inflammation. I would ask about something more long term as far as steroids go as well as possibly seeing a specialist. I would ask about an orthopedic specialist.