Last period was jan 2, period is now 3 days late, had sex jan 12-13, used condoms, im stressed over school & work, back hurts & have cramps, am i pregnant or is cycle changing?

Some thoughts...: 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… Home urine pregnancy test is accurate 3 weeks after sex or 1 week after missing a period. Pregnancy is a common cause of missed periods, if sexually active.