Why wont me female pet rat which is a baby wont let me pick her up?

i just got a female pet rat the the pet store only one though just one rat and about 3 days after that when i got to pick her up or put my hand in the cage she bites at me and runs around and gets into a jerk like really fast so could anyone help me i know shes a female and they are hyper but is she playing or is she scared?