Both Jazz and Scout, who were shelter cats, had diarrhea for the first several days or so they were here then it firmed up. I attributed it to the change in diet and stress (and clavamox for Jazz). Later I discovered they both had tapeworms and I didn't figure it out for a little while. I had the pleasure of finding something that looked like rice - ewww! By that point they had firm BMs and no diarrhea so I don't think it is an automatic sign of worms. I got them about a year apart but the scenerio played out the same in both cases.