The simple answer is stop the biotin and see what happens. Unfortunately the supplement market is unregulated in the US, though I don t know where you are writing from. In the US contaminants, varying dosages from label even within a lot are common. Some supplements contain prescriptive meds such as steroids which can affect menses. Biotin by itself is not associated with menstrual abnormalities. Iron(common contaminant), large doses of lactose (milk sugar often used as a binder in pills) can cause GI symptoms. Sometimes lower GI pain from gas or irritants is difficult to separate from uterine cramps though they shouldn t cause irregularity. If stopping biotin doesn t solve the problem then it is time to be checked for infection, abnormal paps, thyroid or prolactin lab abnormalities, all of which are common causes of menstrual abnormalities. Depending on age and persistence an ultrasound and or office biopsy of the womb may be indicated.