Sounds like she's trying to suckle on the end of your finger, if that's all it is she'll grow out of it. If she's not biting hard or drawing blood then I'd just let it run it's course and wait for her to get bored with it, tapping her nose can make them hand shy, so unless she's latching on don't do it, unless you want a ferret that messes about in warrens when you try to pick her up. Try one of the anti bite products on the end of your finger before you touch her, she'll soon realise your finger tastes bitter and stop doing it.