What does black brown period discharge followed by red discharge with large white tissues mean? should i be worried? i am a 18yo on birth control

Miscarriage? STD?: Any new or abnormal vaginal discharge and any abnormal vaginal bleeding should be professionally evaluated, and you have both. This sounds like you might have been pregnant and had a miscarriage, but that's of course unlikely if "birth control" means hormonal contraception (pill, implant, etc) and you've been on it more than 1-2 cycles. An STD is possible. See a doctor ASAP.