It could be anything or nothing. As long as your cat is acting fine otherwise, eating, drinking, potty-ing, playing normally and no scratching/itching or acting like in pain, I wouldn't worry too much. However if your cat is doing/not doing any of those things out of the ordinary then I'd make an appointment at your vet.

All of mine are spayed/neutered. Bear likes to yowl a LOUD MA-ROOOWW at the ceiling when he's bored and wants attention, his brother Pooky likes to wander through the house and cry softly maaarrroOOw when he's looking for where the people went, and if the old girl Dusty has a nightmare she wakes up suddenly and goes Murauph, muraphhh, maaarouuuph, until someone pets her and tells her it was all a nightmare. lol