The
kanji for "ki" in this expression roughly means
spirit. Really, the expression makes no
literal sense, since it means "I'm becoming a spirit." Rather, it's translated to "
It's bothering me."
However, "ki" also means tree, although it has a different kanji. And so, one day, when my Japanese teacher said "Ki ni narimasu," we all got confused, and asked, "You're becoming a tree?"