Had it been the sex, I would have totally got it. But it seemed that, in a relative short time, she had not only gotten over Fitz, but fell had over heals in love this Will. I dunno.... I felt slightly disappointed that the whole big reason for her to go back is some guy she met.
That might be reading too much into it. I mean, even if they hadn't become lovers, she wouldn't have just left him there once she was rescued. She'd still try to save him because he's a friend and a partner in adversity, and simply because he's a human being who needs help. So the simple fact that she wants to rescue a person in danger doesn't prove that she's fallen hopelessly and eternally in love with him. Their sexual relationship could've just been an expression of comfort and camaraderie.
Although on second thought, if it were just about rescuing someone in need, why would she have kept quiet about it? Why not immediately tell the others that there was still someone back there? So maybe she feels guilty about her tryst with him, which would indicate she still has feelings for Fitz.