one-sided lower back pain

I had that once, and it turned out it was because my pelvis was out of line. The physio fixed it. It might not be anything like that though. Something that helped me was walking briskly, I found it evenen me out sort of, but you may not be able to do that as you say you have to change your gait. My advice would be to go and see a physio if you can.

Has this built up over a period of time or was it cause by a sudden movement? How long has it been going on for? I hope it works out ok for you.

He probably hit it on the nose with that statement above. But, I would caution against thinking you have anything that was mentioned above, and against trying out the PT exercises people are recommending. I have a degenerative disc condition that sometimes leads to what you're describing (and sometimes MUCH worse), so it just goes to show that it can be one of MANY things causing your discomfort. I would give it some time and see if it passes, but if not, get your butt to the doctor.  I wouldn't rush in to trying some of the PT exercises that have been described above until you're pain-free or cleared by a doc. BUT, that's just my opinion. (Some exercises can cause more harm than good if you're doing them improperly.)

In the meantime, you should be safe icing the area where the pain originates. But, I am no doctor so just take what I say with a grain of salt.

Good luck. I hope you feel better soon.