This might be completely unrelated, but let's give it a shot:

I've noticed that sometimes after spending extended periods of time (in the magnitude of hours) in very close proximity with a significant other (e.g. cuddling, kissing, etc.), I feel groggy, dizzy, and have brain fog/difficulty focusing.

Could this be related to CO2 levels? I've been feeling it today and was wondering if it is hormone related, but CO2 could be an interesting explanation as well - I have noticed my breathing tends to slow down during these times as well.

I've never heard anyone else describe this so if you have any similar experiences, please share so I don't feel so alone with this.