Why does my kitten lie on her back, with her claws out, clawing at the air?

I just got MiMi two days ago. I know she is scared and getting used to her new environment. Almost immediately she found a hiding spot under the sink. (its kinda cute : ))..Anyway, lately she's been coming out and playing, but I have noticed that when I would touch her, her claws would come out. Or she will lie on her back and her claws come out; and she does a clawing motion in the air. What does this mean? Should I back off?