Does your place, by any chance, have paint that isn't made out of non-toxic materials? I recently moved and went back to the old place after a few days, noticing how much different the air was. The new place had certified paint used and it was quite noticable.
I'm not sure, but this CO2 issue is something I've noticed in many different situations, including just being in a car with air recirculation turned on. Once I became attuned to CO2 I started noticing its effects everywhere. It's similar to how, once I experienced my first major earthquake (Washington DC in 2011) I suddenly started noticing all of the very minor quakes that happen all the time but that most people never notice.
