Jerry makes a good point. It just may be boredom and lack of stimulation if she seems fine and plays so hard other times. At 1 year old, she is still a puppy.
Do you, or have you tried a good morning walk? Try to take even 10 mins a few times a day to engage her in play, fetching a ball, etc.
you might try getting her a couple of large, hard Kong toys you can fill with treats or peanut butter to keep her amused.
I assume she is up on ***** and wormings? Is she spay? I hope so. If not, has she or is she due to come in season?
I would not be too concerned, at this point. If any other symptoms develop, a Vet visit might be a good idea.
Please come back and update.
At first I thought pain, but a young dog shouldn't have any arthritic problems, and given the rest of your story I see she is active...doesn't sound like pain. Maybe she is bored. Needs more high activity in her day. Just a guess and think I would not be too worried (whatever that means...depends on the person and all the facts, which I don't have).