Damn squirrel nest in yer chimney.
You have CO detectors, don't you?
We currently have one, in the bedroom. Guess this is a wake-up call for me to get more. Though if they ever get the flue cleaned out, it should be fine for a couple of years. The *rap is packed really tightly and they are having a terrible time getting it out past the bend.
I called the chimney sweeps, but they have the answering machine on "due to a high volume of calls". Sigh