It's important to remember that every individual is unique, and not all people with narcissistic traits will behave in the same way. Additionally, I can't know the specific details of your situation.
Narcissists often have a complex and manipulative way of interacting with others, including their romantic partners. They can display love and affection when it serves their interests or when they want to maintain control over someone. So, it's possible for a narcissist to say they love you, even after breaking up, as a means of keeping you emotionally attached or to maintain a sense of power over you.
In the scenario you described, the narcissist might have used the belief that they got back together with someone else to trigger feelings of jealousy or insecurity in you, attempting to gain an emotional reaction or reaffirm their influence over your emotions.
If you've been in a relationship with someone who exhibited narcissistic traits and this behavior is causing you distress, it's essential to prioritize your well-being. It can be helpful to seek support from friends, family, or even a mental health professional who can assist you in processing your feelings and gaining a clearer understanding of the situation.
Remember that a healthy relationship should be built on mutual respect, trust, and open communication. If you find yourself repeatedly experiencing toxic or manipulative behaviors in a relationship, it may be necessary to consider setting boundaries or even ending the relationship for your own emotional and mental health.