hi. what you may be experiencing in your heel is called plantar fasciaitis. It is an inflammation of the ligament on the bottom of the foot. The heel hurts most in the morning and additionally if tired. You can ice it, take ibuprofen, rest it. Make sure you have good supportive and good-fitting shoes. Try a pad from the store. A heel lift also can help. If it persists with this treatment, a podiatrist can make a custom orthotic. I have been experiencing this problem. I do not have AS and do not know the implications of plantar fasciaitis as to this disease. I am on my feet alot(all day at work). If my shoes wear out, which they actually do about every 6 months or less, I begin to experience increased pain. At the moment, I am home from work on a medical leave. I am resting my feet and have significantly recuperated. Good luck.