Is she generally shy? If she's lowering her head while whapping her tail on the ground, she's likely just shy and not sure what to do with the attention...try quieter forms of praise that involve more physical contact and less vocalizing. If she's lowering her head without moving her tail, or curling her tail around herself or tucking it between her legs, then she's frightened. In the case of fright...how much of her history do you know, and how long have you had her? Many shelter rescues or dogs obtained at older ages have been exposed to abuse, and the effects still linger. On the other hand, if she's a shy dog and hasn't gotten to know you very well, she may be afraid simply because she's not sure what to expect from you.