what could be causing my vit d deficiency?

Some medications can cause Vit D deficiency. So can some illnesses. The time of year may be a factor too . . . if you live where it is getting cold and you are all bundled up now. But . . . given that you are taking a supplement and still low . . . it does sound like something is wrong. What does your doctor say? Here is a bit of info from: http://healthlink.mcw.edu/article/1031002458.html "An initial exposure to sunlight (10 -15 minutes) allows adequate time for Vitamin D synthesis and should be followed by application of a sunscreen with an SPF of at least 15 to protect the skin. Ten to fifteen minutes of sun exposure at least two times per week to the face, arms, hands, or back without sunscreen is usually sufficient to provide adequate vitamin D. "

Just wanted to tell you that the same thing happened to me.  Rheumie ran labs indicating Vit D deficiency, and Rx'd 50,000 units once a week for 8 wks.  I only lasted 3 wks before I started having flare problems.  I discontinued the mega dose.  One year later, another Vit D test indicated even lower levels.  This time I went on 1000 mg daily.  When re-checking it after two months of that dose the lab ran both tests: OH25 and 1,25.  The results were, and still are, confusing.  The OH 25 - which as I understand it, relates to supplement intake - was even LOWER (5).  But the 1,25 - which is supposed to reflect what the body is making - was out of range high (90's) 

I don't really think the rheumie knew quite what to make of it either, but told me to get off the 1000/day.  We re-checked it again 2 mos. later (taking no vitamin D) and the OH25 was slightly higher (19) and the 1,25 was down back in the normal range.  Since then I've been taking a multiple vitamin that contains 500 mg of Vit D. 

Sorry I have no answers - only more questions, really - but at least you know you're not alone in having a reduction in your level after supplementation.