There are various reason why she could be biting:
Birds will often bite to 'test' the perch before stepping onto it, it could be an extension of this.
She may just be playing a little too intensely.
She may be scolding you for moving her or being a little cage territorial.
She may just have a more bitey personality particularly if in breeding condition.
If you are certain that she is feeling confident when on your hand, then you can try to distract her, either by using toys/food or by a VERY SLIGHT shake or twist of your finger to make her temporarily just slightly off balance.
Keep in mind that she might need a reason to be on your finger, if she isn't being given a treat or taken somewhere fun or involved in something she finds engaging, then she'll either fly off or make her own entertainment... which might be a bit of nibbling.