Sounds like you lack flexibility everywhere. You indicated that the pain is in the left part of your lower back (not direct spine) providing there is no shooting pain going down the hip, you have 1. Lack of strength in your posterior chain (low back, glutes and hamstrings). Tightness in hamstrings, weakness in glutes resulting in "QL" (Quadratus lumborum) pain or a strain. You need to start a soft tissue and flexibility program that includes upper thoracic, thoracic and lumbar, that will provide almost immediate relief along with ice, anti-inflammatory and either rest or reduce activity. Hamstring, glute and low back program (this includes transverse abdominis using anti-rotation exercises and no type of "crunch") for strength and drop some weight
