I am late by 6 days but have taken 3 pregnancy tests that come back negative. My last period was Dec 21-25 and I was supposed to start 6 days ago. Last time I had sex (unprotected) was Dec 28 (he did not finish in me and I was not ovulating). About a week before my expected January period I began taking diet pills and jurastically changed my diet/food intake, and began exercising every day of the week. Could that have affected my period starting? Could I be pregnant despite the negative tests?