I'm 27 weeks pregnant and my left boob has been hurting REALLY bad on and off today. It hurt last…

I'm 27 weeks pregnant and my left boob has been hurting REALLY bad on and off today. It hurt last week and a couple weeks before that. No lump or anything. My friends (who have kids) say it's normal, but I was wondering if any of you have dealt with it? And why it hurts so bad? It's only my left boob, never my right one