When you got her a few days ago, she was already laying and there were eggs in her system. The stress of moving will usually cause hens to stop laying for a week or two until they settle in, but the eggs in their system will come out first, so it's not unusual for a hen to lay eggs for a couple of days at her new home and then stop. If this hen is alone, with no other flock members to keep her company, she may well be staying in the nest box because she feels that that is the safest place. Flattening down when you approach may just be her cowering in fear. Of course she could be going broody but I think the circumstances are more likely to be a contributory factor to her spending time in the nest box than her hormones. Hens don't like to be on their own. She's in a strange new place with no flock mates.... my bet is that she is hiding rather than broody.
