Nice problem to have. Most women complain guys finish too soon for them. Seeing as she’s your ex, something had to happen to end the feelings you two had for each other. If you were just as attracted to her as you were in the beginning, then I think she was getting upset over nothing. But, if it was taking you longer each time you had sex, then she may be on to something. Maybe other problems in the relationship was taking it’s toll in bed, causing you to take longer to finish. But seeing as she’s now your ‘ex’, you no longer have to worry about that. Hope your current/next relationship works out better!