I am taking a pick to not give a rats ass, and I suggest other "sponsors" to stop give money to this dairy farm that will never get to sex scenes.The nerves in the nipple react to stimuli, both physical and psychological. So an arousing thought, change in temperature, or something as simple as the fabric of your shirt brushing against your skin can cause one or both of your nipples to become erect.
Take your pick as it could mean something or mean nothing