Dark brown or black clots in menstruation are usually a part of the endometrium (uterine lining) shedding.
It may be due to large clot along with more tissue shedding as you have late cycle.
Sometimes old clot also may appeared as dark brown as biopsy was done last month.
If you have unprotected sex during last month, in that case it may be due to miscarriage.
To be sure about the diagnosis it should be examined and investigated.
Contact the gynecologist who has done biopsy as he/she has knowledge about your uterine condition.
If you need further clarifications, I will be happy to explain further.
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