[TheWriterTX]

She may have thought she was coming down with something and was reaching for medication, not knowing the poison was all around her.

Carbon monoxide poisoning can cause dizziness, nausea, and more symptoms, upto and including death.

[Governor Dinwiddie]

At this point, if I had to bet, I would go with Carbon Monoxide poisoning. That may explain the door being ajar, to let in some fresh air. CO binds tightly to the same spot on the red blood cells where oxygen binds. Even if you are given pure oxygen, it can't help you until the CO unbinds. It suffocates you.