Back pain is pretty common when you start, you need to build a strong core to support your weight while running. Do exercises that strengthen the core such as planks, crunches, and ab presses that advance your running routine by around 10% a week to gradually build endurance and give your body time to adapt. Also check your form - heel striking can lead to back pain, run on the balls of your feet or mid-foot. If all else fails, get checked out by a doctor - severe pain is a serious symptom and you could have underlying physical issues that need medical attention. Until you do, don’t put stress on your lower body or back to avoid further injury.