Hey Ricardo,
A side view isn't the most favorable way to critique a squat, but I can see one thing right off the bat that you back doesn't like.
Watch your lumbar spine at 0:25. Ouch! See that over extension you're doing?Remember, you don't want to arch that area. You want it in extension - but not over extension. If that's a voluntary action, quit it immediately. If it's something you're doing without realizing it, I have just made you aware of it. So think 'clamp the abs' for starters.
Outside of that, the movement itself it not bad. I see slightly more vertical shins than I would if you were wearing heeled lifting shoes versus your current choice of flats. Just an observation.
Looks like your eyes are pretty much straight down. This may work for you, or you may like moving your gaze just a little bit farther forward on the floor. Hard to say. You might have to experiment to find what you like. But this is not causing you pain - just a form suggestion I wanted to throw out there.
Provide a better angle (and your full body in the frame) for a better critique. Hope this helps your back pain.