YOU SAID:

I went to the bathroom and pooped my pants.

INTO JAPANESE

トイレに行ってパンツにうんこをしてしまいました。

BACK INTO ENGLISH

I went to the bathroom and pooped in my pants.

INTO JAPANESE

トイレに行ってパンツの中にうんこをしてしまいました。

BACK INTO ENGLISH

I went to the bathroom and pooped in my pants.