please, I NEED advice

Rei
My period is due about now, so this isn't a surprise, but I got really pissy with my boyfriend the past couple of days.
Both times he has felt guilty and stayed calm the whole time and was very gentle. He has yet to ever even raise his voice towards me. I'm very lucky to have him. I love him with all of my heart and he's just perfect in my eyes.
But I feel awful. I don't like being like that. I don't want to sabotage this. How do I control mood swings like that? I don't want to lose him. I want to be as good to him as he is to me.