Maybe non-MS reasons. It is important to look carefully for non-ms factors contributing to fatigue. The list may include poor sleep quality, mood disorder, additive medication side effects, abnormal blood chemistries, hormone imbalance, anemia. Fatigue as exhaustion versus excessive sleepiness must be distinguished. Fatigue with walking may be a specific concern. It is important to see what on the list is treatable.
Burden of disease. Which means many lesions spread over the brain bilaterally contributes to increased fatigue. Ambient heat can make this worse, exertion and sleep deprivation may exacerbate, infection could be occult and result in secondary fatigue.