My son slept almost exactly the same as your little girl when he was her age. He would sleep 11-12 hours a night and still took two naps a day until he was well over 18 months (usually 1-1.5 hours each).
Now he takes one nap a day (usually 2-2.5 hours a day) and still sleeps about 11-12 hours a night. I think kids are all different in terms of how much sleep they need ... but mine sounds just about like yours (he's two and a half now).
Your daughter is probably worn out from just being a one year old ... it's hard work crawling, playing, learning to walk, learning to eat new things, teething, etc. ;)