Possible disc troubl. It is difficult to tell why, but scoliosis, disc disease, arthritis, spondylosis, strains/sprain can cause back pain. Severe hip pain can cause low back and leg pain as well. Without knowing more detail and without exam, it is difficult to tell exactly what you have. I suggest getting evaluated by doc. Motrin/aleve may help with pain in the meantime. Consult doc. Good luck.
Body Weight. Usually when you work out, the whole body weight puts pressure on the back. As simple as it is. Change the work out to a different modality. For example, if you run on a treadmill and it causes back pain, change to a stationary bike and see if that helps. If that doesn't help consult your doctor. You are 31 only and I wouldn't anticipate any major back problem. Thanks!
Depends on the cause. Back pain may be due to: trauma (fracture, sprains, arthritis), infection, metabolic conditions (gout), structural or biomechanical abnormality, blood clot, or nerve damage (neuropathy). A thorough history and physical exam by a professional and possibly some laboratory and radiology exams may help to determine the right cause and appropriate treatment.