Everything is OK. Or at least it's going to be. I went through the entire class paying special attention to what was going on in my left knee, and I figured out what's wrong. The second to last pose is a one leg forward bend. You sit with one leg extended to the corner of the room. You bend the other leg so that the sole of your foot is tucked into the groin and completely touching the thigh. It makes a 90 degree angle between the calf and the extended leg. Then you put your hands over your head, interlock the thumbs, tuck your chin to your chest and bring your forehead to the knee. The goal is to do this, get the knee locked, and eventually get the elbows to the floor. It's one of the two poses in the set where you can pretty much hang out without using strength, once you are into it.
For quite a while, I've been able to basically get my right leg straight with no problem. The last couple of days, I realize, I've been fed up with my left knee popping up off the floor. So I forced it down. And I overstretched it some. I could feel that quite clearly today. This is both good news and better news. The good news is that there's probably no long term danger to the knee. Now that I know what caused the problem, it should be very easy to take care of it. The better news was how easy it was to pay attention to what was going on with the knee throughout the poses, and learning something from it. That's an aspect of yoga that is really cool. It seems to develop an awareness that was unimaginable to me just a few months ago.
I also had a minor breakthrough in Rabbit today. Rabbit is the deepest forward bend in the series. The goal is to have the stretch on the back go all the way from the back of the head to the tailbone. At the beginning, I couldn't get into the basic setup, and I could feel some stretching in my neck and maybe in my shoulderblade area, but that was about it. Today, everything felt great and I really tried to lengthen on the inhales and go deeper on the exhales. And I could actually feel a stretch all the way down into the tailbone. This is another thing that I would have said would never happen. (Just like I would never be able to touch my forehead to my knees.)
I'm going to have to start being less particular about where I am in the class. I haven't had a really good class on the "hot side" of the room. But so what? Today I arrived 20 minutes early, and the room was already filling up. At least, all of the prime real estate was already occupied. I'm thinking it will be much easier to let go of my preference for certain spots. Ultimately it shouldn't matter, and maybe the best way to make it not matter is just not to think about it anymore.