For the last week, I have been experiencing a minor to medium ache in both upper arms during any physical activity (i.e., walking (various levels of intensity), elliptical, etc.). Note, this is NOT sore muscle pain. With this comes an onset of a feeling of fatigue. This aching is bothersome enough that I have had to shorten my gym workouts. I do not experience this aching when I am at rest. What could this be?