It may be related to some lung problem or it may be related to the heart. With a normal stress test it is most likely not ischemia, but you could still have a structural heart disease, such as an Atrial or ventricular septal defect, With a low oxygen saturation this is the most likely scenario as far as cardiac causes of oxygen desaturation, but this is still less likely than a pulmonary process causing this desaturation and arrhythmia.
On this blood oxygen level issue - I went in for a colonoscopy last week and when I was in recovery it was noticed that I had low blood oxygen levels. The friend who was with me said that they would remove the oxygen mask and watch the levels immediately go down to an abnormal level, then go right back up when the mask was replaced. They said I should have this checked out so I do plan to ask my doctor about it when I go in next month.
In the meantime, however, I would like to learn more about this, so are there any comments out there? I have had a couple other surgeries in the last year and this has never been noticed or mentioned before, so I am definitely curious and concerned.