Well, what you have is leukopenia (deficiency of white blood cells). The most common reason for this would be lab error- which is why I asked you how many times they tested your blood. Another common reason would be a viral
infection which affected the production of white blood cells.
More serious causes would include, HIV, histoplasmosis, aplastic anemia, drugs, lupus, leukemia, rheumatoid arthritis, vitamin deficiencies, just to name a few. However, in these cases, you would be showing other symptoms.
I would get the labs redone in about 1 month. If they are still low, then you need to be worked up for this- the doctor would be a hematologist..