The EXACT same thing happens to me almost every session. I couldn't figure out what it was until I went to my doctor (thinking it was a skin rash) and she told me it was anxiety. I should have figured as much.
But I'm still trying to figure out how to make it stop, too. I've tried makeup that's supposed to calm skin and that's actually helped a lot. When I'm in session though, and my T and I start talking about heavy issues, that's when I can feel my neck starting to itch and my face getting redder and redder. And then when I'm aware of it happening, I feel so self conscience and it's just horrible. Ugh. But on the bright side, if we talk about nothing major, my face looks normal. Haha. Can you can pinpoint when yours starts? Maybe you'll find some answers there. You'll have to let me know if you figure out any other tips on this because it's such a major annoyance!!