Bella becomes a vampire becuz she wants to spend the rest of her...uh....existence with Edward. She doesn't want to be human and age and be 98 when edward is always stuck at 17. Then they go on a honey moon and Bella gets pregnant and since the baby is half vampire half human the baby (Renesmee) is very strong and is killing Bella. When Bella does give birth to Renesmee she is so severely injured that she had to be changed to vampire to save her life.
Yes, she does. because When Edward and Bella get married,She gets pregnant, With a Vampire baby. So that baby is absolutely strong. So while shes carrying it, she's litterly dieing, so she does die, And Edward has to change her, yeahhhh.