Girl Friend And I Had Sex Recently, 1-2 Days After

Girl friend and i had sex recently, 1-2 days after she had redish pinkish brown discharge. Thought it was an early period. Hers doesnt start for 12 days. Last time this happened, she had the same discharge, missed her period, but wasnt pregnant. How do we know its not hormones this time? How do we know for sire without going to a doctor?