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If she is having regular periods usually then this gap in between is abnormal. The passing of clots/black pieces and no bleeding could indicate pregnancy or any other serious cause like PID/endometriosis/adenomyosis/ovarian cysts (if associated with cramps). Get her a thorough check-up done by a gynecologist to look for the root cause. She might need blood test and ultrasound scan done to see what is going on.
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