Hi Random_hero, you say your gf is having counselling for her moods. You also mention that this happened with a previous bf of hers.
Is she only like this with the person she is in a relationship with or does she get moody and fly off the handle in other situations (school/work/family)?
The reason I ask, is I think she might be insecure. As soon as she is shown affection and care, she has to 'test' it. It's like pushing the 'self destruct button' because she feels 'its too good to be true' - an unstable childhood, like maybe one of her parents leaving or death of a close relative could have affected her and now she is scared of getting too attached to anyone.
If that is not the case, it could be hormonal, not the period part, the part of growing up that happens in your head when you realise you are becoming an adult. I've yet to meet anyone that doesn't admit to being a pain when they were teenagers. It usually involves telling your parents you hate them, slamming doors and falling out with friends (ever seen Kevin and Perry?).
If she is feeling this way, she probably doesn't like herself much, so wonders what you see in her.
The fact that she apologises shows that she knows she is in the wrong,
Maybe you should be a bit stricter with her, when she starts 'having a go' walk away and tell her you will see her again when she wants to be nice. Or see her less often, so that she looks forward to seeing you and feels less like taking you for granted.
Sounds like you really like her. Good luck.