I'm 38 weeks pregnant. i have a constant dull low back ache and period like cramps. also have had some minor contractions. is this labor?

Maybe,maybe not: You might want to discuss your symptoms with the ob's service & see if an evaluation is advised.They can give you some guidelines based on your discussion.

Maybe early labor : Your complaints sound like early labor. They may get greater or they may subside. Call your doctor if the contractions are every 5 minutes, if you start leaking fluid, you have bleeding, or have decreased fetal movement.