Is anyone else having problems with their cats? I have 4 and recently one got sick,

lethargic, no appetite, swallowing frequently. I took her to the vet and they didn't know what it was, they put her on antibiotics and after a while she seems bettter, now another cat of mine seems lethargic, no appetite and swallowing alot. And the other two are not eating much. They are all indoor cats and have not been exposed to anything. I can't find anything poisonous around the house. We did have the exterminator in a wk or so ago, but he said it was safe for animals and only sprayed outside anyway.  This is really scaring me!! They range in age from 11 yrs.to 3mos.