Oxyhemoglobin/hemoglobin drops: What is the cause? Dangerous?

I am at end stage heart failure from HOCM and recently had a venous puncture done during exercise catherization. My hemoglobin levels were normal (15.8) My oxyhemoglobin were not. When I’m at rest, not moving at all, I get only 66% of what healthy people get (92-98). When I move - even just walking, standing and moving it drops to 36%. I have 3 questions if anyone knows answers-
1) Is this what is causing shortness of breath, brain fog, tiredness and headaches?
2) How dangerous is this?
3) Is this caused by Stiff Heart?
Any info would be appreciated. Just trying to understand what I am going through.
Thanks !
Nini