Oh Runs I feel for you so much. How awful this is to see. Is he still a 'friend' on FB or did you have a peek at his as a non-friend? I'm not sure what your ending with him was like, if it was amicable or not, but maybe the best thing you could do is either block him or, at the very least, unfriend him?
I had EXACTLY this situation happen with Mr Headfuck last year. I was never allowed to his flat, never once met his friends or family, and he grudgingly added me on FB when I got upset he wouldn't (I know now why he wouldn't!!) but then he caused an argument so he had an excuse to unfriend me. It was like I didn't exist in his life (long, messy story). Anyway, very stupidly I still peek at his FB sometimes and see his new woman is happily his friend on there, liking his stuff etc. And also his whatsapp pic is a cutsie cheek-to-cheek couples pic. Sickening. Now I should really take my own advice and block him everywhere, but as yet I haven't. And I keep peeking. And every time I do, like you felt, it's a kick in the guts.
I think some people like to play games, mess with us, treat us badly. I don't know your situation, but if it was a bit turbulent and you felt he behaved oddly/badly towards you, then this could be a way he's triangulating you (and maybe other ex gfs too) with this new woman. If he's a bit of a manipulator/disordered in some way, then you can be sure he's somehow making her jealous too. But, of course, I may be very wrong here and the relationship was good and ended in a decent way.
Either way, try not to look. Try not to wonder whether he was seeing her behind your back or not... it'll do you no good and keep you stuck in the past. Self-care and looking forward is what it's about. You never know when new irons are just around the corner. No good comes of dwelling on the old ones. Hugs.