Possibly broody or thinking about it. Is she showing any other changes in behavior? Puffing up, flattening if you try to touch her, hissing, making a tik-tik-tik sound? What happens if you remove her from the nest box?
You need to decide if you want to hatch/give her chicks or not. If the answer is no, best to break her for her own health and flock dynamics.
Broody jail: Put her in an isolation cage with some food and water, in sight of the others (in the coop if it's not too hot or in/near the run is ideal). A wire cage elevated to air flow under her would be the best option, however I've used everything from a brooder to a dog exercise pen.
Keep her in the cage around the clock for about 2 days. At that time, if she's shows fewer signs of broodiness (puffing up, flattening down and growling, tik tik tik noise) you can let her out to test her. If she runs back to the nest at any point (usually they don't do it immediately, but maybe after 15 minutes, maybe an hour) then she's not yet sufficiently broken and needs to go back to the cage for another 24 hours. Then let her out and test her again. Repeat until she's no longer going to the nest box.
IF the isolation cage is not safe for overnight stay (i.e. sits outside the run, run not predator proof) then put her on the roost at night, and retrieve her from the nest box the next morning and put her back in the cage. It may take a little longer this way but better than letting a predator get to her.