Breastfeeding and formula

Britt
So I went for our 2week check up yesterday and I told Mackenzie's Dr. That for the last 3days I was able to produce enough breastmilk to keep her satisfied finally and had not given her any formula for those 3 days. By the end of the appointment they told me even though I am taking prenatal vitamins still that I needed to get liquid vitamin d drops for baby. That my breastmilk was not good enough all on its own.. So I had asked if supplementing with at least 2-4oz of newborn formula would be sufficient enough and they said yes. So I am a little confused.. I've always heard breast is best, but doc is stating she needs a supplement of vitamin d weather it be from drops or formula. Has anyone else ever heard this before??