My LB has been doing this for a month or so now, and I've noticed it's more when he wants something. Particularly when feeding him he wants more food and he slaps his thigh. I don't think it's anything to be concerned at, more a way of communicating
My son is two. He will smack himself in the face. I was concerned at first, but I realized it wasn't all the time and usually only when he was frustrated. Apparently, it is a common occurrence for our little ones to self-soothe from the frustrations of being unable to adequately express themselves. Do pay close attention and see if you can notice any patterns. Also, pay attention to how hard she's doing it as hitting can be a symptom of other concerns. You can always ask your pediatrician - in fact I advise it.