Could the vitamins I'm taking make my period late?

Zoloft is an antidepressant and can mess with hormones too. But, you sound like you're having PMS. Twelve days isn't that late. I've skipped three months before. If you think you may be pregnant take an OTC pregnancy test. Just take it first thing one morning. The more you worry the more stressed your body will be and stress is a known cause for late periods.