This is a wild guess, but does she do it (the head rubbing) when she does not have a pony tail (top/knot) in? Chances are you could be pulling it too tight to her scalp and that is why she is rubbing.
It is really important to not put it in so tight that it pulls on their scalp or corners of their eyes. I always keep at least a thumbs thickness between the band and their little heads.
For the breakage, you can try using Vellus Satin Creme. One of my girls has really fine hair, and it doesn't grow fast on her head, and it breaks easily. I have been using Vellus and it is helping it stay thicker and not break as easily.
