I Have Copd And My Oxygen Levels Are Usually 92

I have copd and my oxygen levels are usually 92 but today at my doctors office after being weighed and having blood drawn, my level dropped to 74 and that has never happened before. I also have just competed 21 days of radiation therapy for breast cancer. I am not on oxygen and my pulmonary doctor is aware that sometimes my my level will drop to 89 per cent. Just wondering what may have caused this. Edie Fanton