There is no telling or not if it is a flea allergy or what it is. Poor thing. I put Revolution on my feral. I buy it through my vet for one of my indoor cats, but I use it on her, because they won't just let you buy it if they have not seen the cat, technically, but that is how I am able to get around that. Revolution works pretty well. Advantage works really well also, and you can buy that anywhere, it is expensive. Don't use the hartz, sargent, biospot cheap stuff....it doesn't work and it can kill or make cats very sick. I have used Pet Armor though, which is the generic for frontline, and not had any problems with it, and it's only $25 and you can find it at Target, Walmart, and other places. I don't know about putting an antibiotic in their food, I would be afraid to do that with just any antibiotic, wouldn't know how much to give her or what kind. You have to be really careful when medicating cats, and make sure you are giving them something that is safe for them and that it is the right amount for their weight, etc. I would put some flea treatment on her for sure though, you won't hurt her if you use Advantage, Frontline, Revolution, those are the safest flea medications, and definitely steer clear of the cheapo ones in the store....that is the last thing she needs if she is already sick and having problems. Thank you for trying to help this poor helpless creature, you are very kind to do that.