Lower back pain may(!) be related do the sacroiliac joint or lumbar spine, since it is very likely that the whole symmetry of your hip, pelvic, spine and muscle tissue has changed since OP. If you had scoliosis prior to OP because of LLD or pelvic obliquity (like I did) the pain may be the result of this state "suddenly" changing.
If it's just a muscular thing, you could try to take a tennis (though that may be too soft) or lacrosse ball (better!), or even a baseball or golfball (that my be too hardcore), or even a doggy-toy rubberball and try to put some pressure on the area for a minute or two, then start massaging in very small movements. You can do this lying down on the ball or start manually while seated, or sit on the ball (but NOT directly on the operated side!). It's difficult to get this 100% right just by texting it out. If it's just a muscular issue, the muscles should react by relaxing when pressure + small movements are applied. It will feel a bit (just a bit!) painful at first, but then the pain must slowly recede. This may take a minute or two. That's when you start making these little movements. If the pain does not recede - it's something else and this small exercise won't acutally help.
I did that with 2 lacrosse balls to soothe muscular pain in that area (lower and upper back) at hospital, while being kept on my back after surgery. (Just stuffed the balls there between matress an back then kept shimmying around a bit with my pelvic or shoulders).