People's behavior and motivations can be complex and varied, and it's challenging to pinpoint a specific reason without more context. It's possible that she may have had personal issues or unresolved conflicts with your father that influenced her behavior towards you.
If you're seeking understanding or resolution about this situation, consider having an open and honest conversation with your dad to gain insights into their past relationship and the dynamics involved. It may also be helpful to communicate directly with the ex-girlfriend if you feel comfortable doing so.
Remember that everyone has their own experiences and perspectives, and sometimes misunderstandings can occur. Try to approach the situation with empathy and an open mind, and focus on finding a constructive way to address any issues that may have arisen.