My period was due to start on sunday and today is wednesday,it hasn't started. could i be pregnant?

Maybe. Maybe not.: Pregnancy is the most common cause of a missed period. If not pregnant, other causes of a late or missed period include: excessive weight loss or weight gain, increased exercising, emotional stress, having an illness, traveling, medicines (especially birth control pills), hormonal disorders, etc… A home urine pregnancy test is very accurate when done a week after missing the start of a period.