Period came today, but it was 8 days late

How come my period was 8 days late? I went to the doctor 5 days before my expected period and they did a vaginal ultrasound to check for cysts because I came in with terrible abdominal pain. Do you think perhaps the prodding around in my lady business could have caused the lateness? Also, usually the first day of my period really isn't too painful, just annoying. But right now it feels like my lower spine is being hammered with a metal spoon from the inside and cramps are very painful. Any ideas on what is causing this?