O I fear ye are poisoned, my handsome young man!" "O yes, I am poisoned: mother, make my bed soon, For I'm sick at the heart, and I fain wald lie down." (18-20)
Well, that's that. Lord Randall's betrayal by his treacherous ladylove is proven here. though the clues have come in bits and pieces. He seems more affected by heartbreak than by poison, which might explain his reluctance to admit the truth until the end of the poem.