First of all, you should undergo one blood test
for beta-HCG and TVS scan to confirm/ exclude pregnancy at the earliest. Some times late period with brown discharge may indicate early sign of miscarriage
In exclusion of pregnancy, other reasons are hormonal imbalance, vaginal infection
, stress/ anxiety, thyroid problem, weight gain/ extreme loss, irregular eating habits, psychological factor etc.
Need to investigate properly to pin point the diagnosis by complete hormone assay, thyroid profile, blood sugar & prolactin
level, vaginal swab test etc. Consult with your gynecologist for proper guidance and medication.
Avoid stress/ tension, take healthy diet with Vitamin & iron supplements
, control body wt, maintain genital hygiene and take sound sleep.
Be well and take care.