Is there a difference in the sound of her growls?
It could be that a growl when you pet her is her way of saying, “Thanks! I needed that!”
And if she growls when you quit, she could be telling you that she would rather your hand keep doing what it was doing!
Listen to her ~~ she won’t lead you astray. Honest! ;o]
Oh, it’s all bluster...
She IS a Rottweiler, so she must keep up the pretense of toughness. That’s what I love most about these dogz...uppity with the ability to back it up.