Sometimes when your milk comes in, your breasts can become painfully full and hard, and it can be difficult for your baby to latch on. Keep trying to feed, as that is the best way to drain your breast. To make it easier for your baby to latch on, you could try expressing a bit of milk before she feeds, either with a pump or by hand-expressing. Try expressing in a nice warm bath.
Many women find alternating hot and cold compresses helps ease the pain of engorgement. Cold Savoy cabbage leaves from the fridge are ideal, as are hot water bottles wrapped up in a towel.