Help! PD is nipping when my hand is in cage!

I bought my PD a month ago, she is about 4 months old. two weeks ago she became tame and there was no more biting/aggressive behavior. A few days she started this thing where if you stick your hand in her cage, she comes at you and starts nipping at your fingers. It isn't hard, but I'm scared she eventually will. Once I start petting her she rolls on to her side but when I stop she starts the nipping again. If I just stick my hand in the cage and move around she will follow me, trying to get a hold of it to nibble. I don't understand why she started this!!