I'm trying to get a better understanding of chord theory. I have notation on a song that is written with 2 sharps thus I place it in the Key of "D". The song is "All My Trials" (traditional folk). The chords are D Am D G D Em A D. My question is since the 5th of the Key is A (V major), how is it that we have an Am. I thought the minor chords would have been under II and III and sometimes VI. Are there exceptions to the rules around Chords in a Key?

TB