It seems that Cressida was truly at point non plus, but she, like several Meyer women before her, is a witch. She has been secretly studying and honing her craft for years. She has practiced little bits of magic - tricks of light and shape and so forth - but has never performed a large piece of work before. However, after a week of deliberation, she decides that now is the time. She works a love spell, crying all the time. She knows that Lawrence will not truly love her, but as her love for him had faded after she saw who we truly was, and she really had no other choice, she performed the spell.
And it worked - Lawrence proposed to her the next evening, in front of his entire family. His father and sisters were shocked and his mama was apoplectic with rage. But Cressida - and her child - were safe.