Last Wednesday during warmups I noticed my back felt... tight. Something was wrong but I had no idea what, so I persisted and did most of the calisthenics (skipped the ones which were obviously lower-back intensive) and then did class (which ended up being positional sparring for the next hour.)
One of my larger teammates offered to go a little heavier with me than usual because he was concerned that the local brownbelt who I outted might have other means of shutting me up in mind. Of course, given that this person in question is a brownbelt, I'm probably screwed anyway-- but despite my back I accepted his offer. And because my back was out, I couldn't even upa, shrimp, or escape with even my usual mediocrity. So, I cowered and protected my face and generally endured. Which was what my teammate wanted to see and show me-- even injured, even against someone twice my size, I can survive.
I really can't pinpoint what I did that cause my back to go out. It's never done this before, and generally I think I have pretty good core support and fairly strong back musculature. But whatever it was, my back did NOT like it. By Friday I was really challenged to get out of bed or stand up from the toilet. I had to miss a friend's baby shower on Sunday, and today I'm still hurting. I will be happy if I feel like training tomorrow night; today is out of the question.
I'm going to set up a massage, hope that helps. Otherwise I've done NSAIDs, hot showers, even tried muscle relaxants. Foam roller and swiss ball have not improved things.