You’d be a natural where I dive in Norcal. 48 to 53 degree water year around. The coldest I ever saw it was 42 and warmest was about 60.
You didn’t get CO poisoning. I’ve been CO poisoned and it was a three day ordeal. Also, the sick feeling didn’t hit me until I was out of the environment for about forty minutes. It was one scary sumbitch I can tell you.
You might have gotten a CO2 overload but I doubt it because that usually happens when someone shallow breathes due to being nervous.
I’ll bet you got a little O2 high from deep breathing combined with nitrox. Hyperventilate and you’ll get a dizzy high feeling, that’s an O2 spike.
Are you in generally good shape, young, healthy?