I've missed two periods.?

It's impossible that I could be pregnant because I've been in a relationship for 5 months. I've always been regular and i can't think of anything out of the ordinary like weight loss or stress. My moods are driving me up the wall, I've never been so grumpy and snappy.Would really like to know what's wrong with me. I'm 20, 5' 8" and size 8 .