18 weeks pregnant, getting very emotional?

I am 18 weeks pregnant and here lately I have been getting really upset really easily. My boyfriend and I had a small disagreement, and the next thing I know, I'm yelling at him telling him I hate him, which is SO far from the truth! I feel so bad! I just got so angry. I told him I was sick of it all and got in my truck and left. I ended up driving around and coming home. He seemed absolutely bewildered and didn't say much, which only angered me more. We had a stressful weekend of traveling and visiting family and it seems like I just snapped. When I woke up in the middle of the night after crying, I barely remembered what I was so mad about! Is this normal? And how do I help my boyfriend understand my outbursts :(