It's a plugged duct, the tiny stone might be grain of thickened milk or fatty-looking milk.
You can try using heat & gentle massage before nursing-
- Warm compress. Try using a disposable diaper: fill the diaper with hot water (try the temperature on your wrist first to avoid burns), squeeze the diaper out a bit, then put the inside of the diaper toward the breast. This will stay warm much longer than a wet cloth.
- Basin soak. Fill sink or bowl with hot water and submerge breast in water while massaging the plugged area toward the nipple.
- Hot Shower. It can be helpful to massage in the shower with a large-toothed comb. The comb should be drawn through a bar of soap until it is very soapy and then used to gently massage over the affected area in the direction of the nipple.
Nurse frequently & empty the breasts thoroughly.Aim for nursing at least every 2 hrs. Keep the affected breast as empty as possible, but don’t neglect the other breast.
When unable to breastfeed, mom should express milk frequently and thoroughly (with a breast pump or by hand).
Hope the blocked ducts will clear soon!