Back contractions??

I'm 40 weeks and 1 day, have been losing my mucus plug for 5 days now, and last night (still continuing), I've been feeling horrible. My back feels like it's having contractions. I don't know how else to explain it. It hurts a little bit in my lower abdomen but the pain is so great in my back it doesn't really bother me. What could this be? Just discomfort or contractions?