Unless your pullet is experiencing an abnormal surge of hormones, she shouldn't be broody unless she's already been laying regularly for at least four weeks straight.
Another reason for a pullet or hen to want to hang out in a nest box is that they are being tormented by the rest of the flock. To determine if this is the reason in your pullet's case, put her with the flock and observe for a while.
If the flock descends on her, pecking her on head and back, and she tries to tuck her head and squirm out of their reach and they keep following and pecking her, then you will know why she flees to the safety of a nest box.
Now, if your pullet is being tormented, you need to discover why. It may be she's sick or has an injury, both of which can cause the flock to turn on her.
All that said, it is quite possible to have a pullet go broody at just seven months if she began laying at five or six months and has laid around a dozen eggs total between the onset of laying and when she began to go broody.