False Alert
Lots of cat owners tend to worry too much as soon as their cat behaves slightly different than usual. But different behavior doesn't necessarily mean that the cat is in heat.
Quite often cats act like "mad" when, for example, a new cat invades the neighborhood, the weather changes or because you are acting differently. Sometimes even cats just wish for a little attention and want to make you know it.
Play it safe
Therefore make sure that your cat's unusual behavior is also "in-heat-behavior" before you see the vet immediately. When you still aren't sure after a few days you can take your cat to the vet and make him take a vaginal swap. With that vaginal swap vets can find out about your cat's hormone balance and if she's probabyl really calling.
Go, see the Vet
As soon as you are 100% sure that your cat is in heat despite its spay or castration you have to go to your vet immediately, because in most cases cats like that still have hormonal active ovarian tissue inside their uterus.
This ovarian tissue triggers hormone releases that make the cat think that it's in heat.
"Lost" unfertilized ova can lead to the forming of cysts inside the uterus and terrible infections that can cause a lot of pain and - in the end - even death.
Two Methods
Hormone Therapy
The vet has two methods to help our cat. The first one and seemingly easier one is to give your cat hormones via pills which make the cat think it had been served. This triggers the ovulation and "ripe" ova are harmless to the cat because they can leave the cat's body.
This method, though, isn't foolproof because hormone therapy never is totally reliable.
A cat getting this therapy will still be in heat twice a year and have to be treated with the pills the rest of her life.
Surgery
The second, seemingly more dangerous, method is to make your cat have a surgery in which the remaining ovarian tissue has to be found inside the uterus and removed. It is indeed a fiddly task for your vet because the tissue can be scattered all inside the uterus.
We, of course, wish you and your cat a lot of luck and hope that it's just a false alert!
Picture CC 2.0 by slowdown