YOU SAID:

my girlfriend asked why i was breathing so heavily, I wasn't.

INTO JAPANESE

私のガールフレンドはなぜ私がとても呼吸していたのかと尋ねました。

BACK INTO ENGLISH

My girlfriend asked me why I was breathing very hard.

INTO JAPANESE

私のガールフレンドはなぜ私が非常に激しく呼吸していたのか尋ねました。

BACK INTO ENGLISH

My girlfriend asked me why I was breathing very hard.