I Bent Over A Truck Bed Thursday Morning To Reachfor

I bent over a truck bed thursday morning to reachfor something. Had a loud pop under my left breast which sounded like a gun. It hurt when it happened. Then it went away only hurt when I layed down then got up. 2 days later today it hurts when I breathe or stretch my left arm. When I sneeze it hurts. When I breathe it hurts. When I take deep breaths it hurts even more. What could it be and should I go to the dr.