Over stretched lower back.
So I was pulling myself into a folded in half position, legs straight, pulling on a railing in front of me. After the significant pain set into my lower back the next day I remembered Coach advising against this in a Journal article somewhere.
It felt fine (like stretching), but was obviously not smart. I will not repeat this mistake. It was hard to sleep for about 3 days and I felt kind of crippled. Bending or going up or down stairs sort of took my breath away. After 3-4 days it went away. No amount of stretching felt good and it felt very different from sore/tight muscles. Definitely felt like inflamed tissue close to my spine which pushed against nerves with any bending. Flexion hurt more than extension.