He hit me...

So I was fooling around with my boyfriend and I bite his stomach like I usually do but this time I must of bite harder than usually because he slapped me in he back of my head like super hard. I was actually kind of upset and started to tear up because it hurt to the point that I had a headache for the rest of the day. He's never done anything like this before but idk what's going on with him lately but he's being extra protective and being mean about things. Like he will freak over certain things I wear and he's never done this.