A Fly, A Mysterious Fly

There was a fly in my house this evening. I know that may not seem bizarre to you all, but it's winter. Don't flies die or hibernate during the winter? I mean I know that today we had quite the heat wave and got up in the 40's (I went out without a coat and felt just fine!), but did the warmth revive the flies prematurely? I think the warmth teased the insects into thinking it's spring.