Your cat is suffering from "Pyometra". Many cats go into heat when they suffer from this disease. Cats go into heat several times a year. It is quite hard to say without examining when she is in heat. Cats can have multiple heat periods and each period can be up to 5-14 days. This is natural process in cats and no one can really predict it.
My cat goes into heat one week on, one week off though out spring and summer when mating season is prime. In the off season I am not sure, since she is still young and has not been mature long enough for me to know. I read that this is a normal cat cycle for some queens
Usually every 6 months but should not be bred in every heat season.
My cat go into heat 5 times a month