I can speculate on some possible reasons why your ex unblocked you after getting into a rebound relationship:
Curiosity: She might be curious about your life and what you've been up to since the breakup. Unblocking you could be a way for her to keep tabs on your social media posts or online activities.
Guilt: She might feel guilty about how things ended between the two of you and unblocking you could be a way to alleviate some of that guilt. It could be her way of reaching out indirectly without having to initiate direct contact.
Closure: She could be seeking closure or trying to understand the reasons behind the breakup. Unblocking you might be a step towards resolving any lingering questions or emotions she has.
Loneliness: Starting a rebound relationship doesn't necessarily mean she has moved on completely. She might be feeling lonely or vulnerable after the breakup and unblocking you might provide some comfort or familiarity.
Regret: It's possible that she regrets blocking you in the first place and now wants to undo that decision. It could be a sign that she's reconsidering her actions.
Friendship: Some people choose to maintain a friendship with their ex-partners after the romantic relationship ends. Unblocking you could be a sign that she is open to the idea of being friends again.
Remember that these are just speculative reasons, and the only way to truly know why your ex unblocked you is to communicate openly and honestly with her. If you're still interested in maintaining a connection or understanding her motivations, consider having a respectful conversation with her about it.