I just bought a 8 year old Mare.?

Before I bought her I took her out for a ride with her owner on another horse & she did fine so I bought her & have had her in my pasture with two other horses and every time I take her out to walk her since she jumps around and throws her head. Is she just still getting used to being in a new place? Everytime I go out in the pasture she lets me walk right up to her and loves too be brushed wich is all I have been able to do with her as of now. So my question is....is she not as broke as they "claimed" or is she just nervous in her new home still? I have had her for about two weeks now. Thanks for the input!