She may be nursing simply because she wants to nurse for comfort. Although that may make you uncomfortable (to nurse w/ no milk supply) many moms do it when they loose their supply for various reasons (adoptive moms, moms with supply issues, pregnant moms, etc). Your baby may decide that she doesn't want to nurse after you loose your supply completely, she may decide that she doesn't like the taste or she may want to continue through out your pregnancy.
Do you want to Tandem Nurse? If not, you may want to wean DD sooner rather than later in order to keep her from getting jelouse of the baby nursing. If you would consider it, you can be lazy about weaning and let DD decide for herself over the next few months.
HTH, I will try to get back to you if you have any questions.
Melissa