Yoga injury

went to a new class that was supose to be a gentle beginners stretching class, and it was, on my back a lot. Folded my knee towards my chest with the other leg between the knee and my chest.

Nothing happened that evening but about 24 hours later, I felt the pain in my lower back and then I couldn't move without pain for the next five days.

I used ice, heating pad, stopped going to the gym. Now not sure if part of my problem is an old mattress or the new seat in my new to me car...meaning something else.

The difference between this class and the other yoga classes is that the length of time that I held the pose, these poses were held for minutes, I may have stretched too long

I have a new adventure to figure out what I need to do to heal and figure out when it's ok to go back and exercise.