ResMed S9 AutoSet mode makes me tired

I got my S9 from my sleep dr in CPAP mode pressure 8. I started getting air in my stomach and vomiting and so I reduced the pressure but left it in CPAP mode. Now I have it set on AutoSet mode with min-pressure 4 and max-pressure 8. The Sleep quality display shows 0.0 AHI, but I am noticably more tired these days. What's going on? What should I do?

I am interested in knowing if this is typical or not! I just got my resmed vpap 25, and after 2 days of using it, I feel more tired than before. Not sure if this is an indication that I am finally sleeping and now my body has to recover from yrs of oxygen deprived sleep.

If you register and list your equipment in addition to the S9 and a little background on you and your sleep test, maybe someone can offer some insight to your problem.