Hello and welcome.
Your cat really needs to go back to the vet. There is a credit card for vet as well as human medical expenses called Care Credit. Go to
http://www.carecredit.com to learn about it. Or, maybe you could borrow some money from a friend or relative
There could be any number of things going on with your cat, but really, only a vet examining the cat could tell more accurately what might be wrong. When you say her 'behind' looks red, brown and irritated, what exact part do you mean? Do you mean around her anus, where poop comes out? If so, she might have a problem with her anal glands, and needs to see a vet about that. The anal glands could be impacted, or infected.
When the vet said the cat had allergies, what did he think she was allergic to?
Offhand, hair that falls off that is not part of normal shedding could possible be due to allergies, or hypothryoidism. Or poor diet of low quality food. What food are you feeding her?
We can come up with some ideas about foods to help figure out what she might be allergic to, but we need to know what you are feeding her now.
You said when you adopted her she was sick. What was wrong with her at that time?
From what you said, I don't get the idea that your cat is dying. Has she lost weight? Can you feel her ribs easily as in it feeling too bony?
