Either she is tired or she is ill -- it's your call; watch and wait, or take her in to the nighttime clinic.
It may be a reaction to being without other cat companionship for the first time in her very short life.
8 weeks is still very young, she is still very much an infant. Try mixing a little people tuna or salmon with her kitten food. Don't use very much, as fish can be for kitties like crack is to people. A little bit of boiled, mashed chicken is good, too.
Tiny beings need tender care to adjust to a new situation. Some kitties do well quickly, others need more time. Be gentle and patient.
Like any infant, kittens do sleep a lot (older cats, too).
Good luck!