WELCOME, Linnea:
I am sorry that You are going through the first stages of AS; these are the worst.
Yes, it is AS because bowel symptoms and SI inflammation are so closely associated through AS and these things are in proximity, one to another.
Check ribcage pain--costochondritis, also plantar fasciitis, and welcome to the club!
Strongly suggest Carol Sinclair's book "The New IBS Low-Starch Diet" with foreword by Professor Ebringer.
You have certainly come to the right place,
John
