It's probably your sacro-illiac joints. There are a few positions I suggest trying out that may help with the pain. I get on my knees in bed, kneel over and put my head higher than my tush, and make my hubby press right where it hurts, sometimes alternating the sides he's pressing on. Sometimes it helps readjust my pelvis so the pain is gone. A heat pack there helps too. Second thing I do is go lay flat on the floor with my knees bent, and alternately rock my pelvis back and forth using my legs. Sometimes the rocking motion will help those joints slip back the way they are supposed to be. Plus sometimes just being still and kind of rolling my pelvis back so my lower back is flat against the floor helps. Actually at first it really kind of hurts, but after a a few seconds it starts to ease and then feel kind of good. One of those things where it feels SO good... after it stops hurting. LOL
Those are how I cope with my picky sacro-illiac joint until I can see the Chiropractor. Hope you find them helpful!