probably sitting on a nerve. typically one gets severe back pain if the baby is pressing on a nerve. i would recommend calling a doctor's office. any assistant can answer. you can also call an emergency just to confirm if this something to worry about - they will bring you in if they are not sure. (i called once when i had numbing down my left arm - and when my son ate some plants i knew nothing about - they are very helpful)
i am 37 weeks pregnant what are the sharp vaginal pains that make it seem like my baby is stabbing me from the inside?