Zithromax can be difficult on the liver/gallbladder in some people.
It is not so likely to cause diarrhea like other antibiotics...say Clindamycin which is used in dental treatments often.

So I wonder if you are having a bile acid/gall bladder issue?

Call your doctor if you have concerns. But if your doctor is not acquainted with Psyllium use in these situations, he/she may not know the correct answer for you.

Mystery diagnosis had an interesting story about an overactive gall bladder issue that caused terrible diarrhea. Incapacitating diarrhea. The answer was using a sequesterant like the link I gave you.

This is the subject....http://ibs.about.com/od/causesofibs/...a-Syndrome.htm
And I suspect it is more common than suspected.
Zithromax has liver complications associated with it... and it may be your problem. If you get sicker when you eat, and feel better fasting... that is one sign. When you eat those bile acids come raging down, and if there is nothing to digest, then move into the bowel and digest IT.

Since the meds for colitis are not doing much for you...please consider this.