It depends on how bad your IBS is and what type of stress stimulates it. Mine is fairly mild and has never been much of a problem on the job. It's funny, but the type of stress I used to experience as a NICU staff nurse didn't trigger frequent trips to the bathroom -- although I was very careful about what I ate when I worked.
Other types of stressful situations tend to be more of a trigger for me -- which are sometimes a bit of a problem for me in my current role that is a combination of staff development and administration. For example, I recently gave an important presentation at a major nursing conference. Unfortunately, my presentation wasn't until the last day of the conference. I really couldn't enjoy myself very much in spite of the nice hotel, great food, etc. because I was so nervous and my IBS was acting up!
The best you can probably do is wait and see how you do in your student clinical rotations. If it's not a big problem there, it probably won't be much of a problem while working after graduation. If it is a big problem during clinicals and you can't find a good combination of medication and diet to control it, then you may need to re-think your career choice.