Tightness in lower back at hips

Kate • EDD: 07/26/2016 with a baby boy!
I'm 15 weeks pregnant and I'm getting this annoying, but not painful tightness in my lower back right above my hips. It started as one side and now it's both. Just wondering if anyone has had this before. I'm going to call my doctor tomorrow just to make sure, but I figured for peace of mind for tonight someone may have had this as well.