Timid 10 month old GSD

Question
I just purchased a 10 month old female GSD. When I went to see her she came right out of her kennel and came up to me eager to make friends. We played in the a yard and she was very focused on me and seemed very confident. So I bought her. She was great on trip home- very comfortable in the hotel room, not spooked by snowmobiles in the parking lot and willing to go up to strangers in the hotel.  This is great I thought! However, when I took her to the vet last week she was very time with the receptionist when she came over to pet here and the vet had to fiver her multiple treats in order to examine her. I've had a friend over to the house and she was aggressive when the friend came in the door and then was very shy with her the entire visit. Any ideas on why this sudden change in personality and how do I work with it?  The breeder says she has been socialized and was participating in a puppy agility class.

Answer
This sounds like a lack of socialization and the Shepherd's bred in distrust of strangers.  Many breeders think they are doing an adequate job of socialization, but are failing miserably.  When you went to visit, she still had the reassurance of the breeder's presence.  It will take time, perhaps up to a month before she really bonds to you and trusts people because you trust them.  

Control your emotions and make a show of positive greetings when people come.  Have them offer her treats and avoid eye contact, letting their teeth show in a smile, or petting her head.  This will take a lot of work over a period of time.  She may never be as friendly as some dogs are.