YOU SAID:

When you realize you are not wearing pants, but then realize you were only wearing white shorts.

INTO JAPANESE

パンツを履いていないことに気がついたら、白いパンツしか履いていないことに気がつきました。

BACK INTO ENGLISH

When I realized I wasn't wearing any pants, I realized I was only wearing white pants.

INTO JAPANESE

パンツを履いていないことに気づいたとき、白いパンツしか履いていないことに気づきました。

BACK INTO ENGLISH

When I realized I wasn't wearing any pants, I realized I was only wearing white pants.