She may be adjusting to the new home and being away from mom and littermates as was suggested. Of course a vet visit is a definite must to make sure she checks out okay. When Tasha was that age, she would play for awhile, crash and sleep like a log for a bit, then up and play for a bit followed by more snoozing . Her first night home she seemed more interested in sleeping though and it seems like it took her a couple days to really adjust and become herself. I hope all is well with her.