Back pains or labour?

kay
I woke up today with terrible lower back pain. It's constant so can't time them it gas eased off a little but still there or I have just got used to the pain over the day. How do you know the difference between normal back pain or if it's early stages of labour? I'm 38+5 and my midwife said at my last appointment that my baby is back to back. Opinions please :)