P.M.
Sassiness seldom emerges out of nothing at all. I would imagine your daughter is upset by her home life being undone – even if your relationship with her dad was stressful or angry, she still feels a strong emotional impact from the divorce. And kids are known for acting out when upset – it's simply what they do, and they don't have enough life experience to consider what their other choices might be.
She could be picking up that "tone" from playmates or older children she sees, or even from you. Do you ever hear yourself (or other adults in her life) using phrases like these with your daughter?
Cut that out right now, missy, if you know what's good for you!
Get your butt in here right now!
If you think I'm going to put up with that, you've got another think coming!
Stop that whining/pouting/sassing this minute (…or I'll pop you in the mouth/on the butt)!
If you don't get your stuff picked up, it's going right into the trash!
Do you think I'm made out of money?
Shut up, or I'll shut you up!
How many times do I have to tell you to ______, anyway?
Okay, that's it, I'm done with you.
I've been frequently amazed to hear parents who habitually speak to their kids rudely and disrespectfully complain about their children's manner of speaking. I'm not saying you do this, because your daughter has other reasons she may be sassing, but you might consider whether this could be part of the problem. We can be amazingly deaf to our own verbal delivery, especially if we're tired or stressed. (Just like kids, actually.)
Kids imitate the behaviors they see and hear modeled, and their parents are the strongest influence in that regard. We want politeness and respect from our children, but if we're not treating them consistently with politeness and respect, they'll have a much harder time getting there.
May I suggest you read the book How to Talk So Kids Will Listen, and Listen So Kids Will Talk. It gives really practical advice on how to respond to kids by not denying the feelings they have. Amazingly, children can often resolve their own unhappiness once they believe you have really listened to them.
My best to you.