I've been doing a lower spine/pelvis stretch routine since forever - had something similar pop out in my lower back and was racked with pain. Couldn't do much except curl up in a ball on the floor. Basically stand sideways on the fireplace hearth with one foot on it, relax and lower the other foot/leg/hip, letting that side rotate fully downward. Hold that for 10-15 seconds, then pull/rotate that hip up and hold it for 10-15 seconds. 3-4 reps, switch sides and do it all again another 2-3 times. Will probably need a day or two to mend before you can do this, but it gives me great relief. If I start to feel pain or weakness down there, I can generally do this a couple times a day for a few days and avoid any more drastic pain or immobility.
Back pain sucks. Mend well, friend!