Oxygen levels

I recently saw a post asking about people new to using oxygen. The question was why is it that I feel breathless sometimes even when my oxygen level is good. I've been monitoring my oxygen levels and blood pressure since starting on supplemental oxygen several weeks ago. I found that sometimes when I'm breathless, my oxygen levels are just fine, but it's my heart rate that is raised – telling me that my heart is having to work harder to distribute the oxygen through my blood. So I make sure when I feel breathless to put my oxygen on for a small amount of time until I'm feeling better and then my heart rate goes down. I have seen my cardiologist and gone through chemical stress test and echocardiogram, and everything is fine with my heart – so I know that I don't have to worry about my heart. But I also do not want to put a strain on my heart when I need oxygen delivered to my body. So I make sure if I feel out of breath to put my oxygen on for a while.