Subtitled: "I won't have an accident, will I?"
Okay, don't laugh. It's a common question and face it, when you gotta go, you gotta go.
Most importantly, no, you will NOT have an "accident". Unless you are prone to accidents already either while you are awake or when you are sleeping, in which case PLEASE mention that to your hypnotist in advance, all the normal warning systems remain in effect. You will feel all the normal symptoms and you won't ignore them to the point of an accident.
If you feel it coming and it's to the point or even beyond the point where it's distracting or is interfering with your ability to stay focused, simply tell your hypnotist you need a break, using whatever words you are most comfortable using. Your hypnotist will bring you up so that you can take care of the problem. When you return, you will go back under quickly and in all likelihood, even deeper than you were.
Relax, it happens and it's not a big deal.