I agree with opus88...the first thing to start looking for is fleas. Given her symptoms, this sounds like she might be allergic to fleas. Do you have other pets in the house? Often times, you can find fleas on the pets that don't have problems with hair loss or itching. The flea-allergic animals (like your cat might be) often are very good at catching fleas so you won't always find fleas on those pets.
When was the last time she saw a veterinarian? If it was within the last year, you could simply call your veterinarian and ask for their recommended flea control medicine (Advantage, Frontline, Revolution, etc). You could give that a try (remember, dose every pet in the house) and see if there are any improvements. If you don't see her getting better, then she should see a veterinarian ASAP.
The weight loss could be due to the stress of fleas (if that is the underlying issue) or she could have other metabolic type of issues (like hyperthyroidism).
sorry for your kitties health problems...it would be impossible to tell you what could be wrong on a forum....its really pretty darn important you try to take your cat to a Vet..like someone else posted many clinics will allow you to pay monthly.
she is obviously suffering with some ailment and possibly one that could be treated easily and help save her from alot of discomfort.....has she had her vaccines? and has she been treated for fleas?...that would be a good place to start looking at.
good luck and let us know how kitty makes out please.