Heart in your throat...

Guess most of you have heard the expression: I got my heart in my throat ( meaning: You almost scared me to death, or something like that)
I almost dare to say that not many of you have heard the expression: I got my heart in my knee. Could someone please explain this to me...It happened to me and I would really like to know why it happened. An unusaual and not really pleasant feeling, and I would so like to know why, what did I do to deserve this?