Suddenly vocal for no apparent reason

I have a two year old, spayed cat who I've had for the last seven months. I adopted her from a shelter. Within the last three-four days she has suddenly become very vocal. She is still eating/drinking/using the litter/does not appear sick. But she has begun plaintive meows which occasionally turn into howls. She doesn't do it for long periods of time, but she does it consistently over the course of the day. She's keeping me up at night, and I'm afraid my neighbors may complain. Once before she has done something similar, but it stopped after four-five days. I never found out why. Since then she has been fairly silent. Anyone have any idea what this is/what to do about it?

There is always a reason, and usually it is health-related. Cats are skilled at hiding their pain from us because they are so low on the food chain. Please get your cat to the vet.

There may be a new cat in the neighbourhood that she's gotten a whiff of and is responding to that - have you seen any at all? The yowling can also happen (along with a lot of crazy behaviour) during a full moon - this is not New Age stuff, it's real to animals, and mine are like clockwork with their craziness then!

Turns out she was in heat. When they spayed her originally, they left a piece of ovary in. She had it removed and is now home recovering. She's really tender and not a happy girl, but she is eating and alert. Poor little thing. I just want to cuddle her, but I'm letting her rest. It's so hard not to constantly pet her.