Tingling sensation out of nowhere

So I was flying today with my non pilot friend on a XC and in the way to the destination he felt sleepy and lightheaded when I opened the vents he was fine instantly so I assumed he felt hypoxia but it was weird because we were at 3,000ft and the heater wasn’t on so it’s an exhaust leak. The weird thing on our way back still at 3,000ft I was thinking about what happened and I got a tingling sensation out of the blue so I opened the vents and I kept my head moving and I went down to 2000ft I felt fine but still felt sorta tired. Idk if it that was hypoxia or just fatigue or my mind playing games on me lol but definitely don’t want to feel that again