Here, you should undergo one blood test
for beta HCG level to exclude product of conception or implantation
bleeding which is mostly associated with brown discharge.
In exclusion of pregnancy, other reasons are hormonal imbalance (due to withdrawal effect of depo shot), vaginal infection
and may be co-existed with stress/ anxiety, thyroid disorder
, irregular diet habit, psychological factor etc.
Need to investigate properly by doing one USG scan, complete hormone assay, thyroid profile, vaginal swab test, blood sugar etc. Treatment depends on underlying etiology. Consult with your gynecologist at the earliest.
Avoid stress, take healthy diet with iron supplements
, control body wt, maintain genital hygiene properly and take proper sleep.