Musical Muscle Memory.. I play many guitar songs by heart, but I don't know the chords, I have to play the song from the beginning and watch my left hand, only it knows the chords.

I've been playing the guitar for years and know many songs by heart, but if I want to write the 'chords to a song' down for someone else, I don't know the chords, I have to play the song from the beginning and watch my left hand, only it knows the chords.