Hello,
My son started doing that when he was 14 months old approximately. He is now almost 2 years old and I rarely see him do that anymore.
It is a start of terrible twos, he is my second child and my first child (daughter) never did that.
Beware, this will last for a while and could get worse. My son also started head banging shortly after that.
Apparently (I found out) this is normal for kids of that age. About 20 % of boys are head bangers and boys get that more frequently than girls. People who have never seen that get surprised at that behavior, but if you ask your doctor he will tell you not to worry and to ignore it.
Behaviors: slapping own face or head or you with hands or with fists, dropping on the floor and hitting the floor with head repeatedly, banging any other nearby object like a wall or a chair or your knee. If I move away, he stops crying, stands up and comes to me and then drops on the floor in front of me or starts banging against my leg. Sometimes when my son would not sleep at night, he would get up and repeatedly bang his head against the crib rail, in a monotonous way - I found out later that's a way of calming himself down.
Cause: saying no, raising voice, denying what he wants, taking something away from him.
This is to get attention and also because of the inability to express what he wants and inability to deal with his emotions at that age.
This is normal, it is the beginning of terrible twos, and you should ignore it, nothing else.
Also, what helps my child is to acknowledge his feeling, like I would let him be upset, but after a while (like a minute) I would offer a hug and pat his back and tell him I know it's hard but it will be ok. Eventually he has learned to come over to me himself for a hug. I continue to ignore him if he continues to want to bang objects or roll on the floor.
Generally, try to avoid situations that cause that, like do not say no, try positive statements instead of negative, for instance say "come play with me" instead of "don't touch that". Try redirection. Don't use timeouts at this age. Don't raise your voice, don't punish.
My son does not do self-slapping or head banding anymore. I sometimes see him get upset and he lays on the ground, but no head banging and no slapping himself anymore.
My son was also evaluated for the Sensory Processing Disorder and they did not find anything. I would not have evaluated him on my own initiative, but my day care provider wanted to.
SPD is not one symptom, it is a collection of symptoms if you google it and read about it. Also there could be other diagnosis and not SPD, but they never include self-slapping as the single symptom. Other symptoms should be present, usually many other symptoms.
So if you just google the self-slapping or head-banging and read about it, you will be reassured, I am sure. By its own it is not something to freak out about.
I would suggest though that your day care provider should be experienced enough to have seen that before so that she knows how to handle it. Otherwise that behavior might be aggravated by improper handling of the situations. You might want to find a more experience provider who had handled many toddlers in the past. Or switch to a day care center - they are great with things like that (I have seen that myself).
I have switched my day care provider and it works out much better for my son right now, I am very happy.
P.S. Please never hold her down physically. And also, she cannot really hurt herself by doing this. It might seem that she hurts herself but even the doctors will tell you that serious self-inflicted injury by slapping herself of banging is highly unlikely. Just make sure she has safe surroundings (like no sharp objects nearby).