Sounds like just growing-up type behavior. At about this age, in a dog pack, she'd be challenging for the "top dog" role, and she may just be bored. And as a Doberman, she's naturally smart, so it sounds like she could use some mental stimulation, as well as the walks and physical exercise you are giving her. Do you do any type of fetch or frisbee games with her? Toys that are designed to mentally challenge her - like the balls with holes that she'd have to roll around to get the treats to fall out ... Maybe look into agility - anything mentally challenging would be good!