Birth control pills are more than 99.5% effective in prevention of unwanted pregnancy if you take regularly on scheduled time.
Here, you should undergo one blood test for beta HCG level initially to rule out product of conception or implantation
bleeding which is mostly associated with brown spotting
Other reasons are hormonal imbalance associated with pill, vaginal infection
with co-existence of stress/ anxiety, thyroid disorder
, irregular diet habit, psychological factor etc.
Treatment depends on underlying etiology. Consult with your gynecologist for relevant investigations & suggestive measures.
Avoid stress, take healthy diet with iron supplements
, control body wt, maintain genital hygiene properly and take proper sleep.
Be well & take care.