Welcome to HCM.
You got two period in August due to hormonal
You did protected sex on 14th Sept that means around ovulation as your last LMP was last wk of Aug.
Now you get dark brown discharge that is mostly associated with old endometrial tissue rather than normal period bleeding.
Possibility of pregnancy
is less according to your clinical presentation.
You can do a USG scan to evaluate underlying uterine pathology and consult with your doctor.
Maintain personal including genital hygiene to avoid intrauterine infection,keep the area clean & dry,wear cotton underwear and practice safe sex
method to avoid unwanted pregnancy.