Hello... Lower back pain while running happens due to faulty posture of running, wrong shoes and even your extra weight can be the cause. you should consult a sports physiotherapist to get evaluated and examined. If running posture is faulty then he/she will educate you about proper posturing and also you can get to know the proper stretching exercises before and after running. Till then avoid running on hard surfaces like road, don't bend forward too much, keeping your head up will help. Do stretching for calf and hamstring muscles.