Hi ALgirl.
You might like to pop over to the rheumatoid arthritis forum and ask this question. The RA mod, a lovely lady called SydneyJo, has both AS and Crohn's, and may be the best person to answer your question.
I know she tends to spend more time on the RA forum than this one, so your best bet for contacting her is to go over to RA and post there.
Hope this helps
Ivy.