Hello! If it was a sore, over stretched kind of pain then you probably went further than your body was fully capable of. That’s happened to me before. If you have or have had back problems in the past, that could be the cause and you should take it more lightly and slow. You might be doing a few poses wrong, but I’ve never experienced back pain from doing a pose incorrectly. I looked it up and saw a few links that talked about back pain from forward bend, then I read Back pain after Yoga on Livestrong and it says the same thing there. So, that might be your problem. I suggest talking with the instructor about the pain and maybe asking them to watch you do the forward bend or any other poses that might cause that. Especially if they don’t walk around during the class helping people do the pose correctly. (: read more..