Healed84,
I do the EXACT same thing when i get too nervous! It doesn't happen at every t session. But if we talk about something very upsetting, it happens. My t told me that it's one of the things she has consistently noticed about me. So i guess she knows when i'm getting too worked up.

It actually happned yesterday at my session.
I think it's embarrassing too. If i know I'm going to have to do something that is going to make me very nervous (like giving a public speech), i wear a turtleneck or some other shirt that covers my chest and neck.
I notice that once the anxiety recedes, the blotches disappear within about 20 minutes.