My dog isnt as playful suddenly?

So our dog,a female Australian Shepard 3 years old, just isn't as playful. She normally jumps up on my mom and chases the ball ;really energetic. But this afternoon, my brother threw the tennis ball and she ran half way there,then came back and started whimpering a lot. Since then,she hasn't chased the ball,she won't jump as high, and she's been laying down as much as possible. She ate normally and no vomit or anything. Well take her to the vet tomorrow,but does anyone have an idea what might be wrong with her? Thanks.