relative weight

Nikki and I took Sage along with us to the grocery today, after having lunch (Sage, of course, only wanted to use the straw to sip at anything in a cup).

Halfway through shopping, she raised her hands to me from her seating position within the cart. Poor girl was sleepy.

So I picked her up and she went out like a light with frightening ease.

But she also got seemingly heavier - my back is still sore.

So why is that? Is there actually some odd relative weight shift accompany a change in state (wakefulness - sleep)? Is it just in my mind? Why does someone squirming in my arms feel lighter?