change of behaviour

recently, I notice my 1 year old shaking at the sofa whenever I come back home.(she used to run up to the door and wags tail upon seeing me.) could it be due to excitement on seeing me? it should not be due to nervousness, because it has not soiled the house or messed up the place during my absence.and I was never hash with her. the shaking stops when I call out to her calmly and cheerfully. once she leaps off the sofa, the shaking stops. has anyone experience this type of behaviour?