What is my momma cat doing??

I have a first time new mom cat and her babies are almost 3 weeks old walking and playing and almost seeing at this point. After the first week she moved her kittens under the bed because we had too many people looking at them but now after a week My boyfriend and I will be sitting in the living room and she will come in with one of her 2 kittens and drop it on the floor and then stay with it leaving the other one (which i think is too fat for her to carry as when she does carry him she has difficulty) behind. Why is she bringing them away from they nest and into the living room with us?