Another great video by Kelly Starrett on how to look at some self fixing mobilizations for low back pain and tightness. All the options for treatment he talks about is what is performed in most clinics by medical professionals.Your pelvis is not a immobile set of bones and it is basically normal for rotations or shifts to occur causing the muscles in your hips and back to get put into strange positions that can cause them to spasm or cause further injury. A simple realignment of the pelvis can often help relieve pain or get rid of it all together.
These moves are simple and worth a shot to try and self fix yourself when little things go wrong. If you do continue to have pain please go get it looked at by a medical professional.