to m1234,
Have you considered that she may have allergies? She may be itchy at certain places and this may be a way she is relieving this. I had a friend whose daughter did a simular thing. She is now in her teens and has been diagnosed with severe allergies. Just a thought,
If this is something that she has been doing for 2 years, why is it bothering you now? Is it more than just rocking? If you've let her do this for all this time and now decide that it is inappropriate she may be confused. Decide if this is something that is just calming for her or sexually related. If the former it seems quite innocent. Ask more questions.