It's challenging to determine the exact reason for your ex's coldness without more context about your past relationship and the nature of the breakup. However, I can offer some possible explanations:
Lingering emotions: After a breakup, it's normal for emotions to take time to settle. Your ex might still be processing feelings of hurt, anger, or sadness from the breakup, which could be contributing to her cold demeanor.
Need for space: A month and 3 weeks may not be enough time for your ex to fully heal and be ready to reconnect with you. She might still need more time and space to come to terms with the breakup before being open to communication.
Fear of getting hurt again: If the breakup was difficult or there were unresolved issues in the relationship, your ex might be hesitant to open up again to avoid the risk of getting hurt once more.
New perspective or priorities: During the no-contact period, your ex might have gained new perspectives or priorities in life, leading to a change in her feelings or desires regarding the relationship.
She moved on: It's possible that your ex has moved on and is no longer interested in rekindling the relationship or maintaining contact.
Communication approach: The way you initiated contact or the content of your message could have influenced her response. If your message felt insincere or came across as attempting to rekindle the relationship too quickly, she might have reacted coldly.
Other personal reasons: There might be personal issues or circumstances in her life that have nothing to do with you but are affecting her emotional state.
In any case, it's essential to give her space and time to process her feelings. If you want to re-establish a connection, be patient and respectful of her boundaries. If she is open to it, try having an open and honest conversation about your feelings and intentions, but don't push her if she's not ready.
Remember that people handle breakups differently, and healing takes time. Focus on your own personal growth and well-being during this time, regardless of the outcome with your ex.