the other thing is to stop, tell them that hurts mummy and wait a few minutes before putting them back on the nipple. Do this every time they hurt you. Tell them no, take them off, explain it hurts and then wait before putting the back to the BB. They will get the idea once the realise, hey, when I do this, mummy stops giving me the num nums and I really like my milk so I won't do it anymore...
They are just starting to learn cause and effect. They haven't worked out that they can make things happen, that they can hurts things etc. Be paitient but firm. They have to learn they can hurt/damage things and that it is wrong to hurt....