Emotional before period.

Alright guys, I'm 21. I have never had an emotional time before I start my period or really even during it. I'm due to start in 4 days. When I was 9 days out to start, I started to get crazy emotional, and today I am the same way and bitchy for really no reason. I have just been having random spurs of just getting upset and crying over nothing and really angry. Please tell me why I am like this!