I've been on the same birth control for over a year, but this month it has made me nauseous, given me many headaches, and i got my period early. why?

Hormones: It sounds like you have some issues with hormones this month. Maybe you missed a pill or two? If you didn't miss any pills, then likely this change is due to changes in your body's hormones. I would give it another month, and if these symptoms continue, consider seeing doctor for a different birth control Rx. Not every pill is the same for every woman.