Odd question

We have only central AC in our house, as soon as it started getting cold out the entire unit is completely off. As I'm laying in bed tonight I hear something running so I get up and walk toward where it's the loudest, and it the unit running and I have cool air coming from my vents. It's not cold, It's just like the fan is on but the entire unit is completely off so it shouldn't be doing anything at all. Any thoughts?