Hmmm, maybe she's decided thats all she needs.
If she is having 12- 12.5 overnight then her day sleep (altho short) the amount in 24 hours still add up to an average amount sleep for her age. If she is waking up happy then I'd have to assume that thats all she wants.
But that sucks for you cause I know what you mean, I so need that time when DD is asleep and hate the days she decides to have just a short sleep.