Ahh can you tell me what you think?

Basically I'm on my period now and I get really emotional and moody. For the past couple of days, I've been particularly upset and my boyfriend of just over a year just sort of says "I know you're not happy" and when he asks why, all I can say is "I don't know". I also feel like when I'm in these moods he gets annoyed at me or gets bored of my constant sadness. What should I do, say or whatever and what do you think??