Given the data you provided (white blood count 5500 in august 2008, 3800 in may 2009 and 3600 now) it seems likely that you might be experiencing neutropenia. Neutropenia is a condition in which there is an abnormally lower number of white blood cells.
The results solely are not enough for definitive diagnosis, because neutropenia might have several different causes that might be grouped in two major groups: affecting the bone marrow which is responsible for white blood cells production or increasing the destruction of the white blood cells.
The decreased production might be caused by aplastic anemia (a severe condition in which the bone marrow ceases to produce all of the blood cells), certain medications, tumors affecting the blood, and hereditary disorders. Please note that a vitamin B12 deficiency might be a simple and easily treated cause for lower white blood cells production.
Increased white blood cells destruction might be present in certain autoimmune diseases where the immune system detects the white blood cells as foreign cells that have to be destroyed. Chemotherapy, which is used in treating cancers, might destroy white blood cells.
As you can see, there are many possible reasons for neutropenia. Certain of them are more and certain are less serious and easily curable. Therefore, you might want to visit a specialist in hematology for additional tests if the blood cells keep staying low. Meantime, check the blood concentrations of B12 vitamin, because if this condition is detected, then it might be easily solved by taking enough B12 supplements in your body.