Why is my horse so sore?

I have this horse on trial to buy her. She hadn't been ridden for a month or so, and after a week of riding her everyday, she is sore all over. She doesn't show it in the arena, but when the farrier checked her, she cringed when he touched her whithers, rump, and her tendons. She is a thoroughbred, and her back is really skinny, so it might be my saddle. I am using a whither pad on her. and I ride english!!