First of all, you do a blood test
for beta hcg & USG lower abdomen to confirm intrauterine pregnancy
as brown spotting
with late period may indicate implantation
bleeding. Early pregnancy test may give negative result.
In exclusion of pregnancy, other reasons are hormonal imbalance, stress/ anxiety, thyroid problem, wt gain/extreme loss, irregular eating habits etc.
Do some investigations to rule out underlying pathology like thyroid profile & other hormone assay etc. So consult with your doctor and take suggestive measures.
Avoid stress / anxiety, take healthy diet with folic acid
& vitamin supplements,take adequate rest and maintain genital hygiene properly.
If you need further consultation with me, then you can visit http://www.healthcaremagic.com/doctors/dr-soumen-patra/63326.
Take care yourself.