Awww..she's young
...it's hard to get her not to act up with the brushing. Try putting her in a "sit"....putting your hand under her collar...and just gently brush one area. Give her one command with brushing...you can use "easy" or "gentle" or whatever you want. Even if you get 3 little brush strokes in before she get's jumpy again... praise.
This is something that gets better with maturity.
What kind of brush are you using?
Gina