Dreaming of an ex having a new girlfriend can evoke a range of emotions and may reflect your subconscious thoughts and feelings about the past relationship. Here are a few possible interpretations:

1. Feelings of Loss or Jealousy: This dream could indicate that you are processing feelings of jealousy or loss. Seeing your ex with someone new might bring up unresolved emotions related to your past relationship.
2. Moving On: This dream could signify that you are coming to terms with the end of the relationship and that both you and your ex are moving forward in life. It may reflect your own journey toward healing and acceptance.
3. Self-Reflection: Your ex's new relationship might symbolize aspects of yourself or qualities you wish to cultivate. It could prompt you to reflect on what you want in future relationships.
4. Fear of Being Replaced: This dream may highlight fears of inadequacy or being replaced, indicating a need to build self-esteem and confidence in your own worth.

Consider how you felt during the dream and upon waking, as these emotions can provide additional insights into its meaning. If this dream is recurring or causing distress, it might be helpful to explore these feelings further, possibly through journaling or talking with someone you trust.