Hmm lemme see, after Summoning Katsuyu to the leaf village which takes ALOT of Chakra to summon her, then was transmitting chakra to heal others through katsuyu (which takes alot of chakra)
then after pein destroyed the village, she gave Katsuyu all her chakra to protect thousands of villagers.
Then in the Madara fight, she activated her Yin Seal as she entered the battle field, then she healed Gaara and Oonoki, Then she activated Byakugou, then she healed damage from a susanoo sword, a couple of yasa magatama's, and then she gave her chakra to oonoki.
And she is Senju and Uzumaki, meaning that she has a long lifespan, and huge chakra reservoirs!!!
But let me Quote something from the Databook,