"Never within a yard of my bright sleeves Had Launcelot come before: and now so nigh! After that day why is it Guenevere grieves?
The speaker insists that Launcelot had never been close to her before – never even "within a yard" of the "sleeve" of her dress.
Of course, now they were pretty close. They were kissing
She asks herself why, after a totally awesome day like that, "Guenevere" should be sad?
is the first time Guenevere's name has been mentioned in the poem,
other than in the title. Guenevere finally identifies herself. (In King
Arthur stories, her name is usually spelled "Guinevere," but you
probably already know that.)