Common for gf to have mood swings and slap during arguments?

Well don't want to sound like I'm making a big deal out of this (or maybe I am and I shouldn't even be talking about this) but I'm starting to wonder if this is even common or normal. I'm starting to think it isn't. I guess if I've chosen to even write about this, it must mean something isn't right and I haven't been feeling too well lately. It's not the first nor second time but on any type of argument when she feels she doesn't get her way out of something. Though just last week, she actually backhanded instead of it usually being a slap, a bit harder.

This is really my first relationship, we've been dating for nearly 2 years and well this is the only problem but she isn't angry for the longest. Things go back to normal after about 20 minutes or so and it isn't often, just sometimes. I know I'm not threatened by her slaps nor her overall but it feels as if I in a relationship where I can't even disagree or I'm trying to not get her to react. Not sure if I'm being treated unfairly or do I play some role in her mood swings by failing to be more understanding towards her? Can it be that I'm not trying my hardest for this relationship to work? I'm I sounding pathetic by writing this?

Derek, it's domestic violence. If you switched the genders in this, would you tell a woman who's husband hits her that she incited his violence? I'm sure you wouldn't. And the same goes for female violence against men.

Her behaviour is unacceptable in any way, shape or form.

I guess you're right but I just never thought this was actually considered DV and still have a hard time with it while if it was vice-versa yes it would be. Here I am, a former 6'2 a bit over 200 lbs HS athlete while she's 5'7 (ok so not small for a girl but yes in comparing to me) and under 140 lbs.

If I knew a woman was getting hit by her husband, I would be telling her to have him arrested and probably be resisting the urge to go do it back to him. I have a strong reaction towards those that abuse their gfs or wives.

Will have a talk with her.

That's abuse. It doesn't matter if you may be big enough that you can take it, that's not the point. She has no right to be physically attacking you. Not unless she is doing that in defending her own self from danger. If you were attacking her, she'd be well within her rights to defend herself. She is resorting to senseless violence because she apparently doesn't know how to be a mature adult and have a healthy and constructive argument with somebody.

So, no, that is NOT normal or okay by any stretch of the imagination.