You may need to come to grips that she is coming into her own sexual revolution of sorts. She is doing things with others because she is doing better. Revell in her doing better instead of the timing of how she is doing things with him.
Is this "doing better" part of your relationship too? Is she now sexually more available for you as well?
Stop looking at what was and look to the future. Sounds like things are going well.. be happy she is happy and happy with you too..