Sorry to hear about your little hamster
I'd offer advice but I can see that a few of my members have already given some great advice and there is not a great deal I can add. I wonder if at this point it hasn't anything to do with the cat? From the way you describe it sounds as though some time has passed since the incident, a few weeks you said? I wonder if this is a seperate problem, possibly because of a low immune system like racinghamster has suggested?
Anyway, good luck to you and your ham, please keep us informed.