It sounds like you are seeing a first heat cycle. This is pretty common behavior. She might walk low to the ground, flag her tail to the side and will vocalize more. You can expect this to go on for about 7 days. Then she will go out of heat. But keep in mind that cats are free ovulators so they can go right back in. They don't have normal, timed cycles like dogs do.
You need to go ahead and spay her as soon as you can.
Be sure and keep her indoors, no matter how much she tries to go out!
Let me know if you still have questions!
Edited by tfalkner on December 3 2005 at 11:15pm