Bad break up

Well,I finally ended it with my gf for good this time as it kicked off massive style sat night.

We were out for her bday with my mate and her 2 mates having a really good night,went out for a fag and some guy was saying she jokingly looked 30.She asked me how old she looked and I jokingly said 13,so she slapped me hard in the face,I mean really hard and it hurt.
She started apologising and kissing me but I told her shes out of order and dont do it again.

Anyways,the night goes on and me and my mate are having a fag outside when she comes upto me again and slaps me hard in the face with both hands again,it fucking hurt.
So I playfully gave her a little slap back,although I admit it was harder than I meant it to be and then she kicks off.

Storms off down the road with her mate so we all start following them and Ive got these 2 ugly old hags threatening to start on me for being a "woman beater",until she eventually tells them the truth that I didnt punch her or something,then apparently they tried starting on her.
So her and her mate storm off again back to the flat leaving me and my mate and her other mate to walk back.

When we get there her mate has a go at her for leaving her so she storms out into the stairwell with her mate following.Then her mate comes back in crying saying shes out there threatening to throw herself down the stairs and off the balcony to kill herself and also slapping and hitting herself in the face.

We go out to check on her before her mate comes back to drag her inside again,they look like theyre gonna have a fight but eventually we all go back inside.
They go into the bedroom and me,my mate and her other mate try to go to sleep in the front room.Her mate is on the sofa with a duvet and Im freezing so I get on the other end with a massive gap between us to share the duvet.
She storms in and starts saying how Im trying to sleep with her best friend,then we start having an arguement the we go into the bedroom to talk.

Shes getting hysterical begging me to give her another chance and Im saying no because I know things wont work.
Eventually she calms down and we have another long talk/arguement type thing before I say I gotta take my mate home.
She asks for another chance but I say no,then she turns her spiteful face on and rips the shirt off she bought me.
The last thing she says when I leave is "sleeping with my best mate huh".

Then these last few days Ive been getting the usual "have fun being a cokehead and pill popper,have a good life as a druggie".
I had a toy sword that she bought for her son she left at mine and she wanted it back,so I asked my friend from work to take it round to her mums as I knew it would only cause me trouble if I went.
I get a text tonight saying I better watch myself because shes gonna get me back for bruising her face.

Now,I know I hit her a bit harder than I meant to but there were no bruises or marks on her face when I left sunday morning at all.
I am a bit worried to be honest.
My mate who saw it all said there was no reason for her to slap me like she did and I only gave her a little tap,if Id done anything hard she would obviously have been on the floor and he wouldve stepped in and sorted me out.
Why do things have to go so shit?