My girlfriend became angry at me today, and hit me. help.

My girlfriend had been in a great moody for the majority of the day, yet around 4pm she started to became more irritable and angry. For two hours she belittled me and mocked me.. It isn't unusual for her to do it to be honest. I figured it was just another one of her anger spells. She then stopped after I gave her a massage; corny but it works. Then, after a while she started to snoop through my phone. I didn't have a problem with this because I honestly have nothing to hide. At that moment, she found an old message. To a female friend of mine, and the message was also sent prior before my relationship began with my girlfriend. Anyways, she became very angry. Angrier than ever before. She sucker punched me in the side of my head and left a nasty bruise. Mind you, I'm an advid weightlifter and used to take karate courses. Not once did I think about hitting her. Yet, she continued to swing at me whenever I tried to calm her down . When she started to pick kick, I left. Please help me.. I love her with all my heart, but am very confused as to what I should do. I can't put up with her anger issues any longer, especially now that they're becoming physical. Please give me some advice, please..
My girlfriend became angry at me today, and hit me. help.
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