Factual error: The poem Isolde recites, John Donne's "The Good-Morrow", is a 17th-century work, which is centuries later than the movie's time period.
Factual error: In the scene where Tristan is making his return from Ireland and the crowd is gathering around him there are two hens in the foreground. One is a Rhode Island Red and the other is a Black Plymouth Rock. Both of these are American breeds of chicken and did not appear until many hundreds of years after this film is set.
Answer: Both or either. Basically what she's asking is, "Are you really mine and no one else's?"