I lost a jill years ago , she reared to 4 weeks old & then I found her out of the nest box , just generally moping about, so i popped her in a small external run, she walked round the edges of the run in a daze , i took her to the vets who diagnosed milk fever, i cant remember what he gave her , but sadly she didnt make it
he said its when they give everything to the litter & just have nothing left for themselves
edited to add i checked & she was given a calcium injection