why does breast stimulation hurt?

well i love getting close to my gf breast. she has been ok with me get there. i gave her hickies on them and i was playing with her nipples and i started pushing on them and she said carefull. and i backed off and i was doing stuff with them.and pushed up on them and started the nipple thing aghen and i said tell me when to stop. she said ok . and i started like licking her and she started crying and said stop and turned over. she 4 gave me latter but why did it hurt her soooooooooo bad