Sansa Stark
Sans
It was simple you just shortened her name
Arya Stark
My knight in shining armor
You called her this because she was your knight. People laughed at her for her dreams but you supported her 100% so you called her your knight.
Daenerys Targaryen
My dragon
It was obvious one of the reasons you called her your dragon. She was fierce, beautiful, cunning, stubborn, yet caring and compassionate like her dragons. So you call her your dragon.
Margaery Tyrell
My Flower
You called her flower because she was as beautiful as a flower. She was from high garden. So you called her your flower.