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If you had been sexually active without protection around your ovulation time, chances of pregnancy cannot be ruled out. The lighter periods, brown discharge, sore breasts, nausea etc., could suggest few of the early symptoms of pregnancy. Look out for development of any other new symptoms if pregnancy is suspected. If you suspect chances of pregnancy, consider visiting your doctor/gynecologist at the earliest for thorough examination (to confirm pregnancy related changes/signs in body) and blood/serum HCG level test (to confirm/rule out pregnancy accurately). The urine pregnancy tests done very early in pregnancy can show you false negative results as the kits are calibrated ones, so unless the hormone levels in urine reach a certain level, they do not show positive results. Urine pregnancy test kits usually start showing positive results after about 20 days post-conception (roughly corresponding to about 1 week after your missed/altered periods). The test should always be done on early morning first urine sample to get accurate results (pregnancy hormone HCG is believed to be in highest concentration in this sample, and you should avoid lot of fluid intake and caffeinated beverages in the night before taking the test). You may consider taking a repeat urine pregnancy test (on an early morning first urine sample) after 7-10 days of altered periods to see if you get positive results, or get it confirmed by a blood HCG test. Be in regular monitoring and follow-up with your treating doctor/gynecologist and report any new/abnormal symptoms immediately. Take adequate rest. Drink plenty of water. Maintain healthy diet.
Take care.