Okay, so as I was giving DS a bath tonight, he did exactly what you described...straightened his legs and arched his back. He subsequently sunk down too far in the water and startled himself. Silly kid
But he was splashing around when he did this. So, like Shiv52 said, I think your LO is just learning how to play in the water and discovering how nice it is to be in the bath, sans clothing/diaper restrictions. I'd say that she's just having fun
If you find that it hard to get her to wind down afterward, have you tried doing massage? I find that helps my LO...er, when I remember to do it
Good luck! Slippery splashing babies are the best!