Pregnant and low in Vitamin D?

I just got a call from my doctors office saying my Vitamin D is low. I am 15 weeks pregnant and had my last Doctors appointment at week 11. My prenatals have vitamin D and I have been drinking a ton a milk and eating eggs atleast every other day. I was shocked to hear it was low. Does anyone know anything about this? I have never heard of needing extra vitamin D. I am really worried as to how this could affect my baby! The lady from the office said they were calling in a prescription for vitamin D supplements. Please if anyone has had this or knows about it I would love to hear what you know!!! Thanks so much!