That's quite normal, tbh... and is the reason you should ever top or mainline an auto - because their veg period can be really, REALLY short and they don't have time to recover. Most of my autos (and they're all I grow), go into pre-flower in week 3 or 4 but I've grown others that don't go into pre-flower until week 5 or 6 and THEY'RE the ones I've worried about! :-D
Good luck!