Pyometra is caused when the secretions produced by a female cat in heat become infected with the cat's internal uterus or with the flowing blood, causing a bacterial infection. Cats suffering from pyometra will have an enlarged abdomen, decreased food intake, and habitual licking of the vulva. In severe cases, they will have fever, vomiting, and lack of energy. If your cat has the above conditions, you should seek medical treatment in time. Severe pus can threaten illness.