When you’re sexually excited, your nipples tend to harden or become erect. In women, this happens because of the release of oxytocin, a hormone that is usually released when women are excited or stimulated. It causes the contraction of smooth muscles underneath the skin, and this causes a pull on the overlying muscle and/or skin. Once this happens, your nipples start getting shriveled up and almost hard and erect, similar to what happens to the penis when a man is sexually excited.