I don't know your ex, nor do I know you, so I can't give a definite answer to this question. I can only make an educated guess, which is that in one way or the other, he or she isn't yet done with you; hasn't "moved on". This can be is many ways; some exes reach out because they want the other person back, others may be looking for some kind of revenge while other others may just miss the other person being around and want to be friends with their ex. If it's an option, you may want to ask your ex what his or her intentions for reaching out to you are.
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