Why is my o2 level jumping back and forth

I have been out of work 2months now in order to try and see if I can bring my o2 level up from 85%. I brought it up to 90-92% on any given day with a lot of cardio and pulmanary rehab. Now I am I week away from work I woke up today my 02 is at 82% and I need oxygen why is that after so many good weeks in the 90% may be I am gonna have to think of another way to make it in life