This sounds like a resource guarding behavior. She growled to let you know that she did not want you take away the resource she had, the bone, when you continued to try to take it away by reaching for it she escalated her message using the nip. Rather than try to take it away from her when she is clearly saying she is uncomfortable with that, try to trade her for it. Offer a piece of chicken or cheese, in exchange for the bone.
With resource guarders the best thing is to give them their chewies in their crate where they can enjoy them without being bothered and work hard with trading up so that they will surrender the thing they are guarding for a treat reward. A good book to read is by Jean Donaldson is called Mine, http://www.dogwise.com/itemdetails.cfm?ID=DTB740
I would also highly recommend working with a behaviorist or trainer who can observe the behavior.