Why is my kitten bellowing?

I have a 4 month old kitten named Deja. She's starting to become very vocal in the middle of the night, letting out a low meowing, I've never heard her meow like this before, but it is annoying me and making me lose a lot of sleep. She is not spayed as of yet, and I don't know if she's going into heat, because she's only 4 months, and I didn't think females go into heat til 6 months. Anyways, is there any way I can fix this, and have her calm down?