yeah thats normal, for some girls at least
after a while the natural lubrication down there starts to wear away, especially during hard play, so after a while it goes from pleasure to discomfort to just plain pain. even if she feels wet repeated pushing on the tissue down there can get kinda painful after a while. same deal when your fingers are inside as well as out.
or if thats not a problem, the tissues also grow more sensitive during a bout of play, so lighter and lighter presure might help that.
you might also try gently guiding her hand down and kinda showing her how to play with herself. maybe it will get her used to more action down there.
or you could try taking periodic breaks in the action. this helps me a lot
I hope this helps.