I am sorry to hear about your medical problems. If there is increased infection and the body cannot fight it, the WBC start going down as the pathogen start engulfing them. So, yes, it is possible to have very low WBC when the infection is at its peak.
Other common causes of low WBC are lupus (this you have and could have aggravated the whole situation), Hodgkin's lymphoma, some types of cancer, typhoid, malaria, tuberculosis, dengue, enlarged spleen or hyper-spleenism, and folate deficiency. Please discuss with his treating doctor to run tests to find the cause.
You need rigorous management of weakness with appropriate lupus control, multivitamins etc. Eat healthy. Drink plenty of fluids like fruit juices, clear soups, broths etc. Ask your doctor what vitamins and supplements you can take. Consult your lupus specialist. Take care!
The medical advice given should not be considered a substitute for medical care provided by a doctor who can examine you. The advice may not be completely correct for you as the doctor cannot examine you and does not know your complete medical history. Hence this reply to your post should only be considered as a guiding line and you must consult your doctor at the earliest for your medical problem.