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If you have got your normal periods last time, it is possible that you had ovulation or egg release from ovary. If you had unprotected intercourse at time of ovulation, chances of pregnancy cannot be ruled out. The lighter and shorter period/spotting could also indicate possibility of pregnancy. Urine pregnancy tests usually start being positive later at about 20 days post conception, and can show false negative results at times. Visit your doctor/gynecologist for examination and blood HCG test to confirm/rule out pregnancy. Blood HCG tests are accurate in confirming pregnancy early (at about 11 days post conception).
Take care.