Talk to your baby's doctor about a vitamin D supplement.

http://www.babycenter.com/0_diet-for-a-healthy-breastfeeding-mom_3565.bc?page=5

The American Academy of Pediatrics recommends that babies who are exclusively breastfed or who drink less than 32 ounces of formula daily receive a supplement of 400 IU (10 micrograms) of vitamin D each day too.