Many times, ITP can relapse due to a bacterial or viral infection or a medication used to treat them. Sometimes there is no known reason or cause; it just happens. How low do her counts go and how often is she tested? If she is not tested when she doesn't have symptoms, it's possible that counts were down, but not low enough to notice. That has happened to me.
Chronic ITP is considered to be thrombocytopenia that lasts longer than a year, which seems to be the case with your daughter. I'm not sure if it would be considered 'relapsing and remitting' ITP or simply chronic. Maybe just semantics?
Good luck with the test results. Please let us know how everything goes.