We're not 100% sure of hypoxia, are we? You're fine "at work" (I assume cabin altitudes up to 8000 ft.), but jump into a 150 and you get lightheaded. This could be any number of factors. You might start changing "scenarios" - a different headset, or no headset, or a different direction / destination, or with another pilot (real good idea) - and see if things change. Try different airplanes, too, if you can. It might just be a mental thing. I sometimes jump back into a Cessna after several months in Boeings and will totally scare myself.