Hi Marcil and Welcome!
All I can think of is perhaps some anxiety. Instead of sitting up, try to focus on relaxing and taking slow, normal, full breaths. Really relax so that you feel as though you are sinking into your bed. And when you feel dizzy, don't let it panic you, just try to float with it like you are on waves on the ocean. Keep your eyes closed so that you can visualize someplace relaxing and pleasant. It is also important to realize you don't have to breath differently with your CPAP, just nice and slow.
Keep us posted!
Being defeated is often a temporary condition. Giving up is what makes it permanent.
Marilyn Vos Savant
That which does not kill you makes you stronger-Friedrich Nietzsche
Friedrich must of had apnea.