Talk to Gynecologist on Vitamin D Deficiency

I'm a mother of two children , 26 years old. My baby is 7 months old and I'm breastfeeding her. Few days back I was having pain in my hands join(which connect the thumb with finger) and fingers, Then doc advised me to do some tests. Including vitamin D . My vitamin D level is 12 i.e low obviously Another test was anti CCP and it's value was 7.1 . Now I'm having pain in my knee joints as well . Plz guide me about it. I'm feeling weakness in my legs specially when I breastfeed my baby. What should I do for vitamin d deficiency?? Any medicine ?? Or I will concern to any doc ?? Thank you I'm advance.