Shew Fly

Yesterday while getting ready in the morning I saw something flying around my bathroom. Since I didn't have my glasses on, I could only see a blob, so I assumed it was a moth. After putting my glasses on I realized it was a fly. The first thing in my head was, I've never seen a fly in the winter? Where did that come from? I didn't even know flies exist in the winter, at least where it is cold. Needless to say I took care of it before it could escape to some other part of the house and get lost.