@Lost22,
First off, you have no reason to feel embarrassed. You have not done anything wrong.
Moving on to your husband, perhaps nothing occurred with the woman. It may seem he spent too much time with her on that day; and you clearly believe he did. More would needed in the way of information regarding what his prior relationship with her was. If they were just "acquaintances" as he claims or if they had ever dated. Has he said where he knew her from? That aside, it is odd that he never mentioned the meeting her while or on his work related trip. Just happening to run into someone from the past, with or without a romantic past, is something I would mention just because of the rarity of such a thing. Then again, maybe it was nothing to him and he saw no reason or need to mention it.
It's good that you have already arranged for counseling. That may well help clear things up and no matter what, help direct you in which direction you choose to proceed.
I wish you the best, and again remind you, there is no reason for you to feel embarrassed. Now breathe deep, and do something to take your mind off of this for a little while.