What Causes Lightheadedness While On CPAP?

Feeling light headed and then everything starts to go black then goes away. Standing or sitting. Started while driving today and has happened 3 or 4 times since. Blood pressure has been averaging in the 1 ten and 1 teens over 60 s for a few months. Only about 2 hours sleep last night. On c-pap for 8 years. Never felt like this or had this happen before.