So, did he cheat on you, with this girl or someone else, and that’s why he’s your ex?
I get it’s often really painful seeing a recent ex with another person, but the toughest thing here is that he is allowed to see her, whether he lied to you about who she’s dating (or not), even if you’re not healed enough to be ok with it. It hurts, but he’s not obligated to wait for you to be ready for him to move on. If he did cheat on you before you split up, I agree he’s a massive scumbag (but that was never about you anyway - many cheaters feel entitled to do it, and/or they get off on other people’s pain. Nobody can compel another person to cheat, no matter what excuse they give you. It’s a choice).
Following him on IG and talking to him is also your choice, but I believe some would class following him on social media as “pain shopping”. Maybe it’s helpful for some people, for a short time at least, I’m not sure. As you’ve found, you can’t predict or control what he puts on his social media pages, only your own reaction to it.
I’m not trying to be super harsh, I know it’s really horrible to see him do that, so I wanna give you a big virtual hug! I hope it starts hurting less each day for you soon.