Hello XXXXXXXXXX,
This is Dr. Christian. When was your cat spayed? What signs of heat is she exhibiting?
There are basically two ways for a cat to go into to heat after being spayed. The first is called ectopic ovarian tissue. Basically, ovarian tissue develops somewhere in the abdomen and then eventually starts producing the hormones involved in the heat cycle. This is a rare condition and difficult to find. The second possibility is that a small piece of ovary was left in your cat's abdomen when she was spayed. Even the smallest piece could eventually develop the ability to produce the hormones necessary for the heat cycle. Diagnosis requires blood tests and the ovarian tissue can be removed but usually only when the cat is in heat, otherwise the tissue is so small that it is impossible to find. I would recommend talking to your vet about the blood tests necessary to diagnose the condition.
Veterinarian
12 years of veterinary experience in medicine, surgery and behavior with exotics,canines and felines