A few things: the second half of the night is a period of lighter sleep and more REM sleep, so your dd may just be dreaming--she may not be fully awake if she calls out then falls asleep quickly. She may also be teething, have the beginnings of a cold or ear infection which is not too troublesome for most of the time but may be enough to bother her during that part of the night. Finally, has there been a change in her routine: a different nap or bedtime, wakeup time, nap length? Her routine may need a bit of tweaking if there have been changes. How long has she been on one nap?