Malinois shaking Questions

My belgian malinois is 4 months old now. And I'm not sure if it's an excitement thing or something else. But whenever I hold a tennis ball her back legs start shaking and then her whole body starts shivering, but will NOT look at anything but the ball. She will ignore all people, dogs in the dog park and food over a tennis ball.

I took her into a dog park with 3 other dogs in there and they greeted her and she stayed with me and ignored the other people and the dogs because of a ball. Should I not bring a ball in order to socialize her with other dogs? When she does meet other dogs, her hair on the butt area stands up and her neck hairs, but her tail is wagging and she wants to see them, but does it really slowly and she gets kind of low in posture. Some dogs she will lick their mouth and other ones

Sorry some pictures double posted.