How strange......
I squeak when any of my animals nip me (the spiny mice are especially prone to 'test biting' on occasion!) which seems to work quite well.
This may sound odd, but do you think she could be slightly brain damaged or something?! I had a rat that bit everyone (including other rats!) and I was convinced there was something not quite right with her. Try handling her with thick gloves so that you don't react when she bites you as most animals bite to be let go of.
I would be concerned all is well health-wise too though as souffle said. Hope things improve.