Growth spurt, you went back to work, and a million other things. And really, it's very unlikely that any newborn that starts STTN that early will continue it forever. I think you really lucked out for a few weeks. Maybe she'll go back to that, but I wouldn't count on it.
ETA: 8 is not too early, it might even be too late, but that really depends on the kid. Mine went to bed at 6:30 at that age. Also, 3 hours in between naps is a long time at that age.