Hi, Moem. Exercise is extremely important for anyone with arthritis, but for those of us living with AS, it's triply so. We have a saying, "Use it or fuse it." There's good reason for that, as exercise and range of motion/stretching in particular will help keep our spines from fusing.
That said, if you're experiencing pain with the exercises given you by your PT, I strongly suggest that you talk to your PT about it. Explain where the pain is and see if some adjustment can be made to the exercise to take that into account. If you're already in pain in an area, it's not a good idea to do aggressive exercises for that area. Light range of motion and stretching is the only thing you should do to areas in pain, and even then be careful not to push it.
Also, I would ask whether the pain lasts more than a few hours, or if it lasts for days. If it's only a few hours, it may be that your body needs to adjust to these exercises. If it lasts for days, then you've pushed it far too hard and need to ease up a little. Again, talk to your PT about the difficulties you're having with these exercises.
People have already suggested swimming in lieu of riding your bike. Very good suggestion. Your local chapter of The Arthritis Foundation might have a list of public pools in your area that offer aquafit for people with arthritis (teachers trained to deal with people with arthritis). I don't know that for certain, but I would be surprised if they didn't have such a list available.
Just as a side note, it's important that we talk to our healthcare providers if some aspect of our treatment isn't working, or if we cannot comply with it for some reason. In this case, it's your PT, but also your doctors, chiropractors, massage therapists, naturopaths - they need to know these things, or they cannot help us find the treatments that work well for us. Just a general thought.
Hugs,
