Hi, and thanks for your medical question on eHealth forum!
Pain in lower back and pelvic area can be a sign of impemding abortion. Hence it is very important to consult your Obs and Gyn immediately. The other possibilities are infection (pelvic inflammatory disease), gas, kidney stone, or uterine prolapse. Since a confirmed diagnosis cannot be given on net please consult your gynecologist regarding this immediately. Hope this helps. Take care!