I measured my oxygen level a number of times during my 1.5 mile exercise. I was surprised to see that my oxygen level increased during my running, especially when going up small hills. The level was 96% when I started, and it went up to 99% during my runs. After I thought about it, I realized that my running was causing me to breath more deeply and more often, and that put more oxygen into my body. I had already determined on the evening the finger-clip arrived that I could increase the oxygen level by taking deep breaths.
So, I decided that my getting tired during my running is either due to my body condition not being very good from the effect of the blood clots and my inactivity for several weeks while I was getting the swelling under control, or my clots haven't been dissolved yet, or both. My oxygen level is pretty good.