Bras protect the nipples from friction from clothing. Maybe that is the entire problem. Just be sure the bra itself is soft on the inside, though I cannot imagine any bra made that is not.
Do you go braless and wear cotton shirts? Sometimes that can be rough on the nipples.
I second the suggestion to find some pure lanolin and use it. I used to get mine at a shop for things having to do with babies.
We have skin on our breasts like any other and it can become sensitive. It can be dry like dry skin. It rubs on our bras so friction happens. Follow your doctors advice and use either a basic cream on them or whatever product it is you mentioned. lanolin is a famous breast and nipple product.