Had sex for 1st time and got period very heavy 3 weeks later, this month it's 3 days late but i just began track a couple weeks ago, why is it late?

Sometimes happens: In a woman with regular periods, some causes of a late period include: excessive weight loss or weight gain, increased exercising, emotional stress, having an illness, traveling, medicines (like birth control pills), hormonal imbalances, etc… seeing a doctor is a good idea if symptoms are persistent, recurring, or worrisome. Pregnancy is the common cause of a missed period, if sexually active.