You said she's been this way for about a week... so to me, this is sort of a 'sudden' change in demeanor.
So next, I would try and find out if anything happened to her... has she had any 'problems' at school? With friends? With a baby sitter? With any relatives or neighbors? Any possible 'traumas' or someone 'victimizing' her, or perhaps her getting scolded at school, or maybe someone making fun of her at school, or could she be bullied at school or ostracized, or picked on etc. Or maybe a friend at school is no longer her friend, or maybe there are 'cliques' at school etc.
The thing is, you NEED to talk with her... without judgment, but to see what is going on in her life. She is so young, to be feeling this way... it must be something really hard for her... or that she is having a hard time coping with, on her own.
Does she see her Dad? or is he not in the picture? Is he a 'nice' and proper Dad? Did this happen after seeing him, perhaps? What has her Dad told you? Anything?
Think... try to backtrack, and see what has happened in the past week...where has your daughter gone, what has she done? Who has she hung out with? Any extra-curricular activities she participates in?
I think, your girl needs a shoulder to lean on... not necessarily just 'having more fun' with you... but just having someone on her side... WITH her who understands or is willing to listen...
Please let us know what is bothering your daughter, if you find out. The poor girl... she is just so young, to be acting 'depressed.'
If anything, ask your Pediatrician, or take her in for a checkup.... and ask her Teacher about any changes in her behavior as well....
I would really try and investigate this and see what may have caused it or what may have triggered it...
ASK her Teacher, for any clues. See, if anything happened at school.
All the best and take care,
Susan