My period was one day late and it lasted 2 days, heavy with clots, i haven't had unprotected sex in the last 3 months, could i be pregnant?

Hard to guess: 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 by 3 weeks after sex.