Emily - With mothers taking mesalamine medications, some of the potential side effects to breastfed babies are GI-related. This includes vomiting and watery diarrhea. From my research, your pediatrician may be right on track in suspecting the Colazal.
I think you have to do a risk-benefit analysis here. You have breastfed for 6 months, so your daughter has already reaped the benefits of the breastmilk. The colazal obviously helps to keep you in remission, so it may not be beneficial for you to go off of it. My suggestion would be to speak with your GI, explain the situation, and see what he or she thinks. It is important for you to be healthy as well as your baby.