She has inverted nipples. They don't get hard and it's very difficult for the female to breast feed. A lot of women have this condition, and most I've met are quite self-conscious about it. That I suppose is the reason why she doesn't let girls out that much. However with that said I'm more into her ass (at least I wish I were).
My high school girlfriend also had inverted nipples but as soon as she got horny they would pop out. One of my favourite things to do was to suck on them until they were nice and perky whilst I fingered her.