I have the same problem I have gained 15lbs in a couple weeks and my lower back has been KILLING me I couldn't finish leg day the other day and I had to wear a belt just to help my dad do siding. My friend is a massage therapist and he had me lie on my back on hard ground and raise my butt off the ground up on to my shoulders so your knees are in your face. Now slowly set yourself back down but make sure your pelvis is rotated forward (stretching your back muscles). Keep your knees up. And while you walk around try to keep your pelvis right underneath you not rotated backward like most weightlifters do. It helps trust me.