has failure rate near about 0.4%. You can undergo one blood test
for beta-hCG to exclude possible conception. It helps to clear doubt.
Other reasons are vaginal/ pelvic infection
imbalance, mental stress/ anxiety, old endometrial tissue/ blood clot etc.
You need to get examined once by your gynecologist if brown discharge continues for another 1-2 days. Take suggestive measures accordingly.
Maintain your genital hygiene properly. Good luck.