You have what we call leukopenia, which basically means a low white blood cell count.
This can happen for many reasons...
When it is a first time blood test result, the first thing to do typically is repeat it to confirm that it is truly low as well as do a breakdown (called a differential) to see which white blood cells are low (ie lymphocytes, neutrophils).
As mentioned, many things can cause a low white cell count, some serious and some not serious.
Medications are one--rarely one component of your blood pressure medication (called chlorthalidone) can cause leukopenia.Autoimmune
diseases, an overactive thyroid
, severe infections are some other causes.
More worrisome would be a bone marrow issue such as leukemia.
Your doctor, as I mentioned, may first just repeat the test. If it is still low, he/she may refer you to a blood specialist called a hematologist for evaluation as to the cause.
Feel free to ask any follow up questions you may have,