Another weird one. Was just moving the (relatively easy) table laterally, and felt a tweak in my lower back, accompanied with pain.
I rest for two days, then went back to the gym, felt the pain worsen and finished the session earlier.
I had these issues on and off for a while. Must be the heavy squats that are the primary cause. I noticed I can sometimes lean too much forward (due to the weight), which can certainly put pressure on the disks, which then put pressure on the nerves.
This all means I might either have to reduce the weight on the bar (by 10-20 percent) to fix the form issues, or change the training program altogether. The former would mean I would not be improving for a while, and the latter would make me lose strength - none of which are optimal. In the end, I might resort to both solutions.
But first - a bit of rest.