Lower left back pain

A sore back is nothing new after a long day of work. I'm a teacher on my feet all day and I'm used to my back being sore at the end of the day. But starting yesterday I am having a sharper pain on only the left side of my lower back when I shift my weight on or off my left leg. The pain is very uncomfortable and making it difficult to walk. I've tried heating pad and ice. I'm 31weeks. Anyone else experiencing this, know what it is or how to make it better?