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Hi this is Dr. Hill. Sorry that your kitty is having difficulties. At that age, I worry about two primary issues with the mouth. The first is severe dental disease. It is not uncommon for older cats to have severe dental disease that causes swelling and bleeding of the gums. The second is neoplasia. There is always the possibility that she has a mass on that side of her mouth. The only way to determine which it is and how to treat it, is to take her in to your local veterinarian for a dental consult. We do a lot of dentistry in older animals. They will likely want to do some bloodwork and potentially X-rays prior to setting up her dental. They will also likely get her started on some antibiotics. The good news is that dental disease is very treatable. Please ask any additional questions.