Pinched nerve or early labor?

This is my 3rd baby but hopefully my 1st natural labor/delivery. My other 2 I was induced. I started having horrible back pains and what feels like contractions. I'm only 33 weeks so its kinda got me wondering if maybe its preterm labor. At the moment everythings 20 mins or more apart besides the back pain. Its constant. Should I call the doctor? Or could it just be a pinched nerve?