You have set the camera up so that the plates obscure a great deal of what you are doing. However, it does not appear that your back is rounding. I can't tell if you are overextending your spine at the top, once again, because of the plates. You occasionally start with your hips too low and you bend your knees at the top of the lift, but those are rather minor and easily corrected. As for your back pain, there's nothing obvious I can see from your video.