I also have a 20-month-old daughter who will, at times, hit herself (usually in the face) or pull her hair. It is on rare occassion, but it happens when she is very angry or frustrated about something. It is also upsetting to me, as it doesn't seem "normal" to hurt yourself (although she is not really doing it hard enough to hurt herself). My daughter is also typically developing, although she is on the slow side with her verbal skills. I think when my daughter gets really upset about something and then cannot explain to me what is wrong on top of that, she gets overly frustrated and this behavior is the result of that frustration. I really don't think that anything is wrong with her, as I'm sure nothing is wrong with your daughter. I calmly tell my daughter to stop, that I don't want her to hurt herself, and that seems to work. I am hoping the behavior stops as she becomes more verbal. I just wanted you to know that you are not the only one experiencing this.