Yeah that happened me too about 2/3 weeks ago now.
I was deadlifting and i felt a small pop in my lower back. It wasn't too sore, just sorta stiffened up over the new couple of days. I thought it was grand after a bit of stretching, went through a lightish back and leg workout and it still felt ok.
Then, while i was leaning over a chair and using a mouse something shifted and there was like a stabbing pain in my back.
Went to a doc to get it checked out, he said something about a small bone at the bottom of one of the vertebraes popping out of place and sitting on top of some nerves on top of my pelvic bone....
He referred me to the physio for a lot of our national level athletes, he did a few tests (o/h squat in particular) and said that it was caused by my right hip flexor firing ahead of my glute and this imblance caused a twisting in my spine as I deadlifted. And that's what caused the injury..
So maybe do some single leg work...??
I'm doing single leg good mornings, single leg bridging and a lot of core work. Also flexibility drills to and get a better ROM in my hips.
Hope this helps....