The mane six and everypony else were born in the middle of the nuclear fallout. The mane six are close friends and so decided to be couriers together. Pinkie Pie is Courier Six. After her near death experience, Pinkie changed. Unnoticeable at first, the wasteland slowly drained the happiness out of her, leaving her as a shell of the pony she used to be.
 
Refusing to go by Pinkie Pie anymore, Pinkamena will have frenzied outbursts in battle or when she becomes angry, leading her to make rash and deadly decisions she later regrets. Joining the strictly unicorn/pegasus group of Starlight’s (Caesar’s) Legion helped her learn to better control her outbursts, by surrounding herself with ponies suffering close to what she has.
 
But out of all the ponies and creatures who either died and/or left her along the way, only one stayed by her side no matter what, even when her old friends of the mane six ultimately turned on her.