YOU SAID:

I did your mother last fortnight

INTO JAPANESE

私は最後の2週間にあなたの母親をした

BACK INTO ENGLISH

I had sex with your mother in the last two weeks.

INTO JAPANESE

この２週間でお母さんとセックスした

BACK INTO ENGLISH

I've been having sex with your mother for the last two weeks.

INTO JAPANESE

ここ２週間君の母親とセックスしてた

BACK INTO ENGLISH

I've been having sex with your mother for the last two weeks.