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If your cat has been fixed, she will not be able to reproduce, and should not be going into heat. When a female is spayed, they remove all the reproductive organs, uterus, everything...
Maybe there's been some sort of change in your home that's causing some stress for your cat? It doesn't take much change to upset a cat's routine. You might want to give your kitty a little more attention to assure her that she is loved.
If you still think she's in heat, you may want to talk to your vet about it.