I recommend getting another topical product from your vet. Advantage normally works, but you can try Frontline or Revolution. Make sure you apply it as directed. You part her fur so that you can see her skin, then apply the entire contents of the tube directly to the skin. DO NOT rub it in, and do not apply it to the fur.
You need to your kitty with Frontline for the next 4 consecutive months (one treatment every 30 days). Then allow her to move freely around your home. There are probably a few flea eggs and larvae in your environment. You can vacuum and wash bedding, but this isn't very effective. You need to wait for the fleas complete their life cycle into adulthood and jump on your cat, then they die because your cat is treated with Frontline.
It is, unfortunately, a long time to wait. Short of hiring a professional company to come treat your home with a dessicant powder, this is the way to clean house.
But why won't she walk around the house? We are not leaving her in one room she is finding a place to hide and staying there. Also, she is loosing weight is that normal for a cat with a flea problem.
I think she's just having an extreme reaction to a flea allergy. If you are worried that it is something more serious, I recommend taking her to your vet for a physical and some bloodwork.
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No, flea allergies cause severe itching, and she can develop serious skin damage and secondary infections...but I have never seen a cat die from flea allergies.
Kittens however, can lose enough blood from fleas (causing anemia) to weaken and die.
Could she have psychology issues?