What Causes Reduced WBC Count Along With Fatigue And Fever?

I am 37 years old with a history of breast cancer in 2009. Last October during a routine blood test they discovered my WBC was low at 3.9. Went to hematologist who said it was benign leukopenia. For the past 2 weeks I have been fatigued, no fever, cold hands, some tingling in my hands as well. Also I do have a history of hypothyroidism and take 50mcg per day.