See below... Leukopenia (decreased number of white blood cells) may be seen in various clinical conditions including infections, congenital deficiencies, chemotherapy and some other medications, radiation, toxins, leukemia, aplastic anemia, autoimmune disease, bone marrow infiltration by cancer, etc. Consulting a hematologist is recommended.
Many things. A whole host of things, from certain viral infections, bone marrow problems, drug reactions to auto immune diseases. It needs immediate attention if it's new to determine cause as well as prevent the potential infectious complications associated with a low white count. Ps what's the rest of your labs eg platelet count and hemoglobin.
Leukopenia. Multiple reasons varying from viral process to something serious, needs to be checked out by hematologist asap.