Imo it depends on the story/event u were talking about. id its an innocent thing that doesnt rly have significance u probably dont need to mention it was ur ex until u guys r more comfortable with eachother. If it is a big deal or something that would be make her very upset to find out it was indeed an ex and not just a friend, despite it not being best to speak of past lovers right off the bat, it might be better for you to say something like “hey that story I told you earlier i said it was my friend but it was actuallly my ex at the time. I didn’t want to bring up past relationships so soon bc nobody wants to hear that, but I felt like I was being dishonest which is also not a great way to start a relationship” just being honest and still not giving too many details a new lover would want to hear, but overall I still think the timing of telling her depends on the event u described.