I bent down and now iv a sore back even sore wen i walk and use my left leg what is this ?

Acute low back pain: Age is the most useful screening factor in low back pain. At your age without any prior history of lbp, most probably musculoskeletal/mechanical in nature. Keep moving without heavy lifting, spinal manipulation, massage therapy, Naproxen 500mg twice a day with meals with a muscle relaxant for 3 days. If no improvement in 2 w, a more in depth neurological exam to r/o a radiculopathy is indicated.