I get that exact burning sensation in the bottom of my spine! Weak core muscles; specifically wrong muscles trying to support the core. I had zero lower abdominals. Pilate's helps a lot.

I have to consciously engage muscles, even after practicing for years (can't afford Pilate's sessions which would make it go faster). Now I very rarely get the burning sensation and corresponding days, weeks, or months or not being able to sit or do much of anything.