As you have excluded all local causes of the bleeding, it is more likely that the spotting could be a breakthrough from the uterine lining; the brownish color also favors that. This could happen when there is a disturbance of the hypothalamus-pituitary-ovarian axis due to several causes like erratic weight loss
or gain, temperamental disturbances, hormonal
disorders, systemic disorders, drug interactions and so on. Please ask your specialist to evaluate completely before initiating treatment. Meanwhile try to alleviate stress
and maintain a healthy weight and eat healthy. Take care.