I don't have experience of false pregnancy in cats but I do in dogs, as my own dog used to get mild false pregnancies. However I think it is a very similar mechanism in both. It is to do with a surge of hormones, possibly Progesterone (?)
In itself, it is not anything to worry about as the situation usually balances out okay when the hormones balance again.
But, a history of false pregnancy can be more dangerous later on in life, when Pyometra or inflammation of the womb can occur (which is a medical emergency) This can sometimes be deadly.
My advice is -unless you seriously need to breed your cat (i.e. you are an experienced breeder (?)....then please get her spayed. Pyometra if it occurs, is sometimes obvious and sometimes not. It is easily missed, and I would think, more easily missed in cats than dogs. It can kill.
Unless it's an emergency, a vet won't spay while your cat is in season. But usually about half way between 'heat cycles'. Ask your vet for advice.
You may be interested to read my blog article which is about Pyometra. Hover over the number "1" after "Blog Entries" under my avatar, then click to read. This article was written with dogs in mind, based upon my own experiences, and the veterinary care received. But I should think it applies equally to cats.
I hope your girl will be happy and healthy for a long time!