Am I crazy or is this not right?

In grilling, I mean asking, my son about his birthday plans, he mentioned that he talked it over with his dad in therapy.
"Um, say that again? Your dad's been going to your therapy sessions?" I believe at this point my hair went curly again.
"I said it would be OK. If he wanted to." Baby notices my hair and tries to cover his tracks.
"It's fine," I lie and run from the room because I wear my emotions like a Lady Gaga headpiece.
The only thing I know about boundaries is in the 257 pages I've digested in the boundary book but this feels like a violation to me. Long ago ex and I decided that these sessions are our baby's personal time and that we wouldn't even asked him what was discussed in the Dr.'s office. Oh, and we split the fee. It seems or it feels like I'm sharing in paying for ex to work on his relationship with our son. I feel pissy.