What could it be? Sleep regression ?

I’ve gotten my almost one year old on a goodish schedule. She’ll go to bed around 9 or 9:30 ( used to be midnight) she sleeps in her crib in a dark room with a full belly and clean butt and normally she’ll sleep until 9 or 10 but right now she’s not going to bed and I don’t know what’s going on we did the same thing we do every night. Her bother (2 years old) normally falls asleep around 10 ( still working on his sleeping )but tonight he didn’t want to go to sleep till 11.

Advice please:)