Anytime anger and physical violence (and this qualifies) are directed at you by a loved one you must see there's a major problem, it's extremely abusive and you should either deliver an ultimatum that it stops and she immediately seek help or you're done. This is zero tolerance territory.
On the whole its an extremely unusual act. Slaps in general are a way of inflicting pain without inflicting harm. Compare that to a fist and the intent of a punch is to do real harm the other person. A slap in the face is usually a physical insult: dismissive, disrespect, and insult made physical. A slap to the chest has a vibe of asserting dominance and ownership and was probably something she learned from her dad.
Don't tolerate this shit any longer. Insist she begin therapy to change her habits and make it clear that if it happens again, you're done.