Low WBC count
It depends on how low the count is.
The usual lower limit of normal for most labs is around 3, 500 wbc/mm3. If you are in the 1500-3499 range, this is probably not causing the fatigue. If it goes down lower, it may make you more susceptible to certain infections, which could cause fatigue. It if is very low, you need to look for a specific cause, and many of these might cause fatigue.