Do NOT power through this! This is your body telling you something is wrong. If you can’t find a solution, drop the lift. Front squats may be a safer variation for your back. There are so many variations on the squat, that avoiding back squats doesn’t have to be the end of all things.
It seems muscular (you said massaging it helped), so I’d try self myofascial release with a tennis ball. Just prop a tennis ball between your back and a wall. Start in the area that spasms. Apply pressure to the ball from your body weight and roll.
Check out Bret Contreras’ 21 Exercises for Injury Free Mass
Cant do front squats due to rotator cuff tear. So that leaves hack squats which also agitate my shoulder. I can do the leg press but its just not the same.
Ive done the tennis ball thing for 6 months straight, it helps alittle bit but is no solution.
My spine feels like there is some laxaty in it. When I get the muscles completely loose with massage/stretching/muscle relaxers it will make popping noises and feel almost worse than when it is spasmed.