DP and I have always maintained some friendships that are not 'couple' based, friends who are exes, friends who re-emerged after many years, and we meet them individually - no problem whatsoever.
However we do not fail to mention protracted contact or plans to be meeting up, we chat about their news an 'oh, I had a message from ExYZ yesterday...they have got a new job" etc. And we are not secretive about plans, and if there was a long journey involved it would be "FormerFriend suggested meeting for lunch halfway, it's near BeautifulNTplace, do you fancy a day out and I can drop you off " etc.
OP - you are certainly not overreacting, and this must be very upsetting for you. HIs attitude is not kind or respectful.
I know you feel fragile, but can you organise your 'case' in your mind, and ask him calmly for a discussion. Be as calm as possible an say that whatever is or is not going on, that there is a problem.
That he is not communicating, making pans without telling you, being defensive and cagey, and whatever the reasons you would like him to be open with you about it.
Do not be drawn into his accusations that you are hysterical, suspicious, simply wrong, or whatever,. just stick like a stuck record , and calmly, to the issues you have raised.
If he will not talk further just say "OK we will have to leave it that you have concealed plans from me, not felt able to be open about your messages, and have felt able to go ahead to the meeting knowing that it would upset me. For me, that is a breach of trust, love and respect in our marriage"
Then up to you: ask him to sleep on the sofa, seek legal advice, ask him if he will go to couples counselling, chuck him straight out...or live with it til the next thing happens. Up to you. It is a horrible situation and the longer he lets it go on, the crueller it is.
No consolation to you at all that so many others have been through similar.