I had low back pain at my SI joint (about where your "dimples" would be on lower back and also pain across my left butt cheek into my hip. I went to a physical therapist.
He did gentle manipulations, gave me strengthening exercises and told me to use ice not heat.
That helped the SI pain but not the butt thing. I got a medula maternity belt and after a week that went away too. I had been in so much pain I couldn't walk.
It's caused by relaxin loosening all the joints in your pelvis. Something starts pressing on a nerve and bingo PAIN. But don't just accept it, find a PT or chiro to help you out.