Does brown discharge that turns red after a week signify pregnancy?

Not necessarily: Pregnancy itself doesn't cause brown or red (bloody) vaginal discharge. Are you sure it isn't a normal period? Or pregnant and now early miscarriage? Any abnormal or unexplained vaginal discharge or bleeding should be professionally evaluated, and you apparently have both. STDs (gonorrhea, chlamydia) are other possibilities and very common at your age. See your doctor soon.