Labor starting?

Ok. So i am due on Friday (April 10th).  I have heard nipple stimulation will help induce labor. So I tried it this morning with my breast pump. About 5 minutes after I started the baby was moving around like crazy.  I had the pump on for about 20 minutes and about 10- 20 minutes later I had a pretty big cramp right under my belly. Hasn't really gone away and goes into my back-pelvis area. Feels like menstrual cramps.    Is this labor? I'm going to take a bath but I wonder how long before I should call my doc?