Back Pain...need advice :(

My baby girl is due Feb. 5 (21 weeks, 6 days). I'm 20yo, 4'11, currently 109lbs. Pre-pregnancy weight 95. This is my first baby. And every evening for about 1.5 weeks, I have had awful upper back pain on the left side. I try walking it out, changing positions, taking Tylenol. .. nothing works. It keeps hurtin til I'm finally able to fall asleep. I need advice as to how to make it ease up..& why it hurts so bad?! It's driving me crazy....