I'll try to keep this brief!
So I'm in my 50s, the DS in question is in his 20s, living at home.
I work in a team with mainly younger women in their 20s, and I'm particularly friendly with two of them, let's call them Ann and Kate. When I say friendly, I mean we go to a dance class and go running together after work, go to the theatre together every so often, etc, and have lunch together every day. Ann openly says she looks to me as 'the mum she never had', and messages me for advice about various things most evenings. Kate and I don't tend to message outside of work much.
I introduced DS to both Ann and Kate, and he comes running with us occasionally.
About a month ago, DS laughingly told me that he'd 'matched' with Kate on Tinder. I laughed and told him that obviously he couldn't date her, as she was my colleague and friend (albeit much younger), and this would make things very awkward for me. Plenty more fish in the sea and all that. He agreed. The next day at work I joked with Kate that they had matched, and said the same to her that it would be so awkward if they dated. She also agreed.
Since then, the four of us have continued running together every week, and things have carried on as normal.
Last couple of Saturdays DS has been out on a date, and was very cagey when I asked who she was. He isn't usually, we are quite close (so I thought).
Naively, I was joking at work about DS 'secret date' and both Ann and Kate were laughing as we speculated who this might be.
Turns out it was Kate all along. DS told me, and has apologised, but I am soooooooo upset. Also now wondering if Ann knew, and whether they have all just made me into a laughing stock behind my back.
How do I deal with this at work? Annoyingly, I have just helped Kate out massively work-wise. I would like to keep my dignity, so won't be making any fuss, I am just feeling very stupid, and very hurt.
Also wondering if I'm foolish for thinking that age is 'just a number' and therefore I could be friends with people a lot younger? (I do have plenty of friends my own age too, btw!)