When is your next session? If you just bring it up and be like, "I was in this support group over Facebook and I got into a disagreement with another person. I acted like an idiot out of anger and come to find out after that this person is also a client of yours. I just wanted to double check am make sure that my poor choices with X won't affect our relationship."

Or don't bring it up unless your T does. It's unlikely she would because she can't legally even tell you X is her client so how could she even bring it up?

I'd tackle it head on personally so I wouldn't be wondering and worrying about it.