SeaMonkey has given you excellent information and I cannot think of anything that I can add to what she has told you. I just wanted to reiterate what she said that this deficiency is seen often with Lupus patients and, as such, try not to over worry yourself about why it has occurred.
Was your deficiency diagnosed by your doctor? If so, did he prescribe or suggest supplements? I, too, take vitamin D supplements due to a deficiency caused by my Lupus. So far, the OTC supplements are working for me!
Please do not stress yourself about this too much, speak with your doctor to see what, if any, meds or supplements you can take!
Peace and Blessings
Look For The Good and Praise It!