Tightening during back pain?

I am currently 38w1d and I have been having really bad cramps in my back. I didn't have back pain with dd at all during labor so my question is..how do you time it? and does your stomach still tighten when you are having back labor? These past few weeks have been so confusing to me. I've had contractions start and stop for weeks now. I'm just curious how you know when it's labor when it's in your back.