Hi Darcy, and thanks for your post and question - it's a good one! Although I am not a physician or a diagnostician, I can answer your concern in a general way. For more specifics as to why this happens in your particular case and, for your level of disease, I would suggest you discuss this with your private physician.
Generally speaking, COPD can affect oxygenation levels based on the poorly matched ventilation/perfusion units within the lung. This can be characteristic of the disease itself. At rest, one's oxygenation level may be normal/acceptable for the patient to be relatively comfortable. In some cases, with the slightest degree of physical exertion, these ventilation/perfusion match-ups are unable to handle the load. Consequently the oxygen level may drop and the patient may feel a level of discomfort and difficulty breathing.
I understand just how frustrating this can be for you. I thought you might gain some additional insight from this article which addresses this topic too: https://copd.net/search?s=oxygen%20saturation%20and%20breathlessness.
Again, you may want to take this concern up with your physician. The doctor will be able to focus his answer (for you), based on your specific case.
Wishing you well,
Leon (site moderator COPD.net)