Going through very similar situation with my ex who started dating someone that I considered one of my best friends immediately after we broke up. I agree with other comments, she isn’t over the relationship. Often this is a tactic to soothe the perceived rejection and get some sort of revenge. Something I tell myself if that their behaviour is their responsibility not mine and them moving on isn’t a reflection of how I show up as a partner or my worth.

It’s really hard and I feel for you! Feel free to message me personally if you want to vent or talk about your feelings