I can offer some general possibilities as to why your ex-boyfriend might have started talking to his ex-girlfriend right after your breakup:
Comfort and familiarity: After a breakup, people often seek comfort and support from familiar sources. His ex-girlfriend may be someone he feels comfortable talking to during this emotional time.
Emotional attachment: There might still be unresolved feelings or emotional attachment between them, and the breakup could have triggered those emotions, leading him to reach out to her.
Nostalgia and memories: The end of a relationship can make people reminisce about past experiences. Talking to his ex-girlfriend might be a way for him to process those memories.
Loneliness: Breakups can leave people feeling lonely and seeking connection. Talking to his ex-girlfriend might be his way of trying to alleviate that loneliness.
Attempting to move on: Paradoxically, some people might try to move on from a recent breakup by seeking validation or a sense of being desired by someone else.
Seeking closure: He may be trying to find closure or understanding about the end of your relationship by talking to his ex-girlfriend.
Reconciliation possibility: It's also possible that he is considering the possibility of getting back together with his ex-girlfriend.
Remember that everyone's motivations and reactions are different, so these are just general possibilities. It's essential to focus on your own well-being during this time and not dwell too much on his actions. It's okay to feel hurt or confused, but try to give yourself space to heal and move forward. If you have concerns or questions about his actions, consider having an open and honest conversation with him when you feel ready.