Vitamin D Deficiency?

My doctor just left a message and said my blood test showed I have very low levels of vitamin D and to immediately start taking supplements. However I drink a lot of milk, probably about a gallon a week, that is fortified with both A and D and also already take a multivitamin with 400 IU (100%DV) of D every day. How can I be deficient? I have an appointment with my doctor next week and will ask her as well but if anyone could offer an answer I'd appreciate it.