Dizzy/headaches, no CO alarm sound

I’m staying in a cabin for the weekend. 11 friends and I, and the wood stove is our only source of heat. I’m campaigning to keep a door cracked at all times, but I’m getting pressure headaches and feeling a little dizzy by the end of a long day inside. Nobody else is feeling anything, the chimney is working, and the CO alarm is working and not going off. Any advice?