I assume the oxygen monitor is of the light type? where 2 different frequencies of light detect the haemoglobin. Even when sitting, this is just a rough guide. I have seen these monitors state 99% saturation yet the patient is whisked off to ICU because they have seriously low oxygen and are hallucinating.
As far as I am aware, these monitors are only good when sitting and relaxed because once you start to exert, blood flow around the body alters, including in the hands. A lot more significantly in some individuals.
The best way for oxygen testing is a blood test (blood gas).
I was given a similar test last year. I had to put the light sensor on my finger and get a reading. It was 99%. I then had to walk around the ward three times, sit down and take it again. It was 94% but soon climbed back to 99%.
A pulse of 115 is nothing to worry about.
I think when walking about a pulse of 115 would be within normal limits for anyone and as far as your 02 levels dropping that also is pretty common when we exercise or expand the lungs.....i would just call the doc and run the question by them for confirmation tho...............