I had the same problem with Taco when I first adopted her. Turns out shes just a talkative/loving cat that seeks attention constantly. She was however spayed at the humane society in early pregnancy, so that might have had something to do with it. I'm not entirely sure tho. I asked the vet, and sometimes not all the reproductive organs are taken out. I don't think its that common, but thats what I was told. It can cause them to go into heat. Just give your vet a call, they should be able to put your mind at ease.
Many people enjoy the companionship of cats and dogs, who were domesticated thousands of years ago. Over time, people have manipulated animal breeding to produce certain physical characteristics, resulting in the different types of cats and dogs we know today. But domestication took these animals out of their natural environment, and their reproduction is no longer regulated by predators or habitat. The result is an overpopulation crisis that can only be controlled through widespread spaying and neutering.