Could pink/brownish discharge be an STD or implantation?

Hello,

Welcome to ehealthforum and thanks for writing

Since you had unprotected intercourse during the ovulation days so possibility of pregnancy is high.But it is too early to have implantation bleeding as after fertilization a woman can have symptoms of pregnancy as early as after a week.
Light bleeding might be just old menstrual blood being shed,any local abrasion,hormonal fluctuation etc.STD cannot just present with light discharge instead other symptoms as pain,itching,ulcers,fever,abnormal swelling etc might also be there.

Relax yourself and just wait for the next periods.If they are missed then get a Urine test done for pregnancy determination.In between if you notice any unusual symptom which is botehring as prolonged bleeding,discharge,pain etc then consider visiting a Gynaecologist for evaluation.

Take care!