What Causes A Missed Period And Sensitive Nipples?

I should have started my period last saturday, and today is wednesday. 2am Tuesday morning I had a gush of blood and that's about it. I have been achy in my lower abdomen, almost like dull cramps but not quite. I start day 1 of my bc pill today and have had no cycle. Boobs are not sore but nipples are sensitive and that's an oddity for me. Advice please...