I just recently re-started doing yoga after my left hip replacement in July. I never had any issues like you speak about until about 2 wks ago when I tried just some gentle yoga. I was in so much pain, I stopped.
I'm still going to P.T & so asked her what she thought. I showed her a few of the movements I do and she said, " That's because now w/ u having AS. u have to remember that all joints, which most connect to the back/spine in some way, are going to feel some pain." She told me to stop what I was doing & instead showed me some VERY easy, gently exercises that really don't seem to do much.
But as she explained, these movements will work out the big muscles & joints we need to use to move. I can't show them to you but I would suggest, if you can, to talk about seeing a physical therapist that knows your issues & show u moves that won't cause u pain. Good luck and I hope I helped, even just a little!