usually nipples harden due to cold whether , or like feeling cold , it also hardens if you are feeling horny , and some girls have said they nipples become sore when they gonna get there period soon , it may also due to the bra , rubbing against it uncomfortably because its too small or too big may cause it to rub repeatedly on the bra , this is what I think is the reason but I don't think this is anything serious .
