Corinne, don't worry much about setbacks, they will reslove as you get further through the process. A cold can go either way with IBS, make it worse or sometimes make it better. Most of the times probably worse though.
Cold meds may upset it also, just fyi.
Stay positive as much as possible, which it seems your doing well and try to make your mind up, you will have a good time on the trip and not have any problems. Then, don't give it any more thought or worry.
Have a good time and I hope your cold resolves by then also.