WIBTA if I messaged a guy I dated’s new girlfriend to warn her off him?

Basically for context I got with this guy we got along really well talked every day, I thought he was lovely took me out on loads of dates told me he was falling for me like it was a bit fast because we’d only known eachother like 2 weeks but yeah I slept with him and then he dumped me the same day and said he was still in love with his ex. It was a whole thing I was upset yano but this was in February and I’m since in a relationship with a really lovely guy that didn’t pie me off. However, occasionally he’d still randomly message me to say hey and ask how I am and normally rant about his ex girlfriend not loving him it’s a whole thing and he’s said that he’d recently been sleeping around and did the same thing to another girl that he did to me. Since then he has gotten into a relationship with a girl that he’s only known for 2 weeks that looks exactly like his ex girlfriend and even has the SAME name as her! I’ve also heard from one of my friends that he was messaging one of their friends while getting with this girl. I definitely do not have feelings for him and I don’t want it to come off like I’m a weird stalker but it’s so upsetting to see all these girls getting used by him. I haven’t really made up my mind if it would be okay to message any of these girls but I guess I just want some opinions. WIBTA?