What are your chances if you are breastfeeding and using one bottle of formula a day of getting pregnant ?

Depends: If you are sexually active, you will likely get pregnant if you try to rely of breastfeeding as BC. It is so unreliable that I consider it no protection at all.

Chances are good: If you are having unprotected sex. Even full breast feeding is not a guarantee of protection. You can ovulate before you get your first period and become pregnant if not using protection. Having a baby at your age is a very big event. Maybe you would be more comfortable postponing further sexual activity until you are settled in with motherhood. If not, do use effective birth control.