Weird Behaviour

I have a female cat who was about six months old when I found her wandering the streets she was also pregnant.
She has had her kittens and been desexed. She now continually drags socks around the house. The kittens are still with us as I cannot find homes and she still washes the kittens and occassionally feeds them.

What is this sock thing?

Can someone please help me?

It may just be a maternal thing. Thats how moms move thier kittens around. However, she may just like the socks. If I leave a bra out, I have one cat who will take it everywhere with her!!