It is REALLY hard to tell at that age. You're going to have to wait until she's older to be able to determine if she has pit bull in her -- and that's a guess at the very best. Without papers, you'll never know for sure.
But, if I remember correctly, that's about the weight my pups were at 6 weeks.
Also, why is she away from her mother and littermates at that age?
The front Legs have some nice Round Bones, and it seems to have a desent Jaw muscle along with some other features. Then again its hard with a picture, and there are other musculer breeds out ther...maybe a lilttle boxer too, But IDK..
If i had to guess at this age, My best "guestimate" would be that it either had some American Bulldog or Boxer, just based on the snout. Based on the angle of head compared to muzzle, There is also a good possibility of Pit.