Could my kitten be in heat?

I have a 4 and a half month old female kitten that I think may be starting to be in heat - is this common so young??
She responds already to catnip (I thought it was not supposed to work when they were not sexually mature yet) and writhes around on the floor like a mad woman, biting the scratchpost and squeaking!!
She also tries to get outside whenever she can and cries at the door but I have stopped letting her out for fear of male cats in the area. She is very affectionate lately too.
she doesnt cry all night or anything, but she is behaving a bit odd recently - does this sound like kitten in heat??
I am going to get her spayed soon if my vet will agree that its not too young (he said six months when I had her vaccinated)
confused
Kittay

Yes! It does sound likes she's maturing and going into a first heat. Some breeds of cats do mature early (some of my American Curls would begin maturing by 5 to 6 months.)

Keep her inside as you are doing, and get your vet to spay her soon If she's a good weight, and you explain all these classic signs of being in heat, hopefully he'll agree to spaying her earlier than he is used to doing.

Here in the US, many vets now do early spay/neuter, and I've seen young females altered at 3 months who did very well post-surgery.