How should I handle this?

My step daughter who I thought I had a good relationship with based on interactions and comments that she has made has apparently told her mother that she thinks I don’t like her and that I intentionally keep her father away from her and her siblings and “hog” him. I was actually extremely hurt by this because she wrote me a letter last year expressing how happy she is that I’m in her life and how happy she is that her dad met me. Her mother was she one that called my partner to tell him that his daughter feels this way and that he should talk to me so that I can fix the relationship. She is 13 years old. Im confused because she comes over all the time, doesnt give the impression that there is an issue. I don’t always tag along and I let the kids have their own time with their dad even at the house and aside from that the kids do spend a lot of time with me while their dad is at work. Not sure how to approach the situation..