I do not see anything to worry,whenever there is bleeding in very small amounts it turns brown or black as by the time it leaves uterus and seen outside there is a time lapse so haemoglobin of blood which is red changes to haemosiderin which is brown.IN your case I see it as may be ovulation bleeding,to be safe do a
TSH,CBC AND
PROLACTIN blood tests and a sonography PELVIS.With the reports if you wish u may consult me.