I don't know where you are but in the U.S. anyone in a position that oversees children is court mandated to report if there are any signs of abuse or anything that could bring harm to a child. This includes teachers, daycare workers, etc. This also includes therapists. When I signed my paperwork for counseling after my DUI this court mandated information was included in the paperwork.
Here's the thing though:
Now she wants to meet before our next appointment to discuss the fact that she may have to report me.
If she's meeting with you beforehand I don't want to make any guesses but it appears she needs to feel that this decision was a one time poor choice and will never happen again. I think that due to your realization of the possible consequences of what you did that possibility is no longer existent and she wants to feel confident about it. It's scary, I know. She's looking out for the best interests of your child who has no choice. You do have a choice and letting her know that you understand how serious this is and that's exactly why you are seeing her will help.
Good luck and let us know how it goes. You're doing a good thing here. I think the fact that you even disclosed that to her shows how you feel about the situation and it will never happen again,