you may have a gas leak 
I know you're joking, but many years ago some idiot fitted the external flue to our gas fire incorrectly so carbon monoxide was drifting back into the front room - we thought we had some weird flu or something, as at various times we had headaches, felt dizzy, nauseous, etc. Noticed it a couple of days later when out in the garden... (the flue being upside down)
Serious bidness...