38 weeks pregnant and VERY bad pain in lower back that doesn't go away. ?

My wife is 38 weeks pregnant and is saying she is having lower back pain. It is a constant pain and isn't going away. It's been waking her up all night. She had contractions the other day and we went to the hospital and ended up getting sent home because the contractions weren't bad enough then. Now she is having these back pains that do not include any pain in her stomach at the moment. Is this back labor or is the baby on her spine or something?