Why do I have Bad back pain?

Im 15 and i live on a farm,this summer we did more hay than normal and I had to stack it. the bales were about 50-65 pounds a piece. When I was lifting the bale I twisted and my back popped and it HURT. I kept working and eventually it went away, one day it came back and I went to the chiropractor to get it fixed, he said that my spine rotated a little? But he fixed it. But yesterday I picked up my backpack from the ground awkward and it hurts again. what should I do?