hello and thank you for asking HCM,
I understand your concern and my suggestion is to visit a gastroenterologist because ankylosing spondylitis
can be associated with other autoimmune disease
, which can affect your abdominal system. For example ulcer colitis
or crohn's disease. So try to ask your GP to send you to this specialist or go by yourself if it's possible.
I hope this asnwer was helpful to you.