I've only had to share a bathroom when I was in the dorm and so when I did that I tried to just go when no one else was in there. But obviously that wont work in your case, I did share a bathroom with a complete stranger who was my roommate for a semester though, all I told her was that I had CF and didn't really explain my toilet issues with her. I tried to go when she wasn't going to be in the room, it was more like an apartment...it was easy cause we had opposite class schedules. Maybe you will get lucky and it will workout that for you!
If not and you are really worried and want to explain it to her then what I would do is maybe give her one of those guides they have at the CF clinic...they usually give them to elem school teachers, its a basic guide to CF.
By college we are all mature, atleast most of us, are mature enough to handle these issues without being childish. Just tell her that sometimes you get really bad stomach aches and might have to use the restroom a lot and just make sure you always have Febreeze on hand. I don't think you will have a problem, you are probably just over analyzing and worrying too much.
Hope this helps!