"O yes, I am poisoned: mother, make my bed soon, For I'm sick at the heart, and I fain wald lie down." (19-20)
Lord Randall's refrain changes a wee bit here. Where he used to claim that he was weary from his hunting trip, he now admits to being "sick at the heart." This so-called sickness seems more important even than the fact that he's poisoned, suggesting that the poor young man is really dying of lovesickness (well, and poison).