YOU SAID:

I fell down the stairs, and now my back hurts.

INTO JAPANESE

階段を下りて腰が痛くなった。

BACK INTO ENGLISH

I got a pain in my back down the stairs.

INTO JAPANESE

階段を下りて背中が痛くなった。

BACK INTO ENGLISH

I went down the stairs and my back hurt.

INTO JAPANESE

階段を下りて背中が痛い。

BACK INTO ENGLISH

My back hurts down the stairs.

INTO JAPANESE

背中が階段を下って痛い。

BACK INTO ENGLISH

My back hurts down the stairs.