YOU SAID:

I am a big girl who wears big girl undies

INTO JAPANESE

私は大きな女の子の下着を身に着けている大きな女の子

BACK INTO ENGLISH

I'm a big girl wears big girl underwear

INTO JAPANESE

私は大きな女の子の身に着けている大きな女の子下着

BACK INTO ENGLISH

I was wearing a big girl with big girl underwear

INTO JAPANESE

大きな女の子大きな女の子の下着を着ていた

BACK INTO ENGLISH

Big girl was wearing big girl underwear

INTO JAPANESE

大きな女の子が大きな女の子の下着を着ていた

BACK INTO ENGLISH

Big girl was wearing big girl underwear