Considering the changes in her sleep patterns and sleep behaviour, etc have happened so quickly are now dramatically different than previously, I would mention it to her doctor.
11:30-12:00 is very late to sleep and when is she having her breakfast, lunch, etc?
You should try to get her back into a normal schedule.........wake her up t between 7:30-8:30 so she is getting between 11-12 hrs a night, and then hopefully she will be tired enough after lunch to have a nap for 1-2 hrs.
My son just turned two last month and his schedule is: he goes to bed around 8-8:30 pm, wakes up around 7:30am, has breakfast at 8am,
lunch around 12-12:30pm, nap from 1-3 pm, and I never let him sleep past 3:30pm, otherwise he won't want to go to bed until very late.
Obviously, there are days when we can't stick to the schedule exactly, but we are basically doing this routine everyday and he is really benefitting from it.
I'm not saying this is the routine you should follow, but it's just a guildeline that you may or may not want to follow. Every child is different and will therefore have different sleep patterns.
A great book to pick up from the library or store is :
[/b]Healthy Sleep Habits, Happy Child by Marc Weissbluth, MD