Is her rear end swollen? If so, perhaps she laid internally. You can take a syringe to remove some fluid from that area, if fluid is yellow and murky it is likely a yolk that was laid internally and will eventually be absorbed.
If the liquid is clear, she might have cancer. Ovaian cancer is fairly common in laying hens. If cancer, she will feel bloated and heavy, but she will have a protruding breast bone due to a reduced appetite. If cancer, she will likely get more bloated and heavier but her breast bone will get more obvious. I suggest weighing her and feeling her breast bone for changes.
