Why did your ex mother appear in your dream?

The question is not clear, since it is impossible to have an "ex-mother." You might have a former mother-in-law, who is the mother of your ex-wife, or you might have a "late mother," meaning that your mother is deceased. You also might have a former step-mother, who is an ex-wife of your father. In any of these situations, the meaning of the dream would depend on what happens in the dream, the feelings you experience in the dream, and what, if anything, you and the "mother" say or do in the dream.