I have started having breast tenderness diarrhea and migraines with occasional cramps. got my period this past monday. what could this be?

Not unusual: Due to the hormonal fluctuations and the inflammatory cascade, it is not unusual to get loose stool accompanied by a headache, breast tenderness and uterine cramping just before the start of a menstrual bleed. You may try Ibuprofen for two days before cycle starts. Your doctor could prescribe a stronger anti-inflammatory with same schedule. Try active pills only OCP.... unless menstrual migraines.