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If you have been sexually active (and not on any form of birth control measures), the chances of pregnancy cannot be ruled out completely. Sometimes urine pregnancy tests can keep showing you false negative results during early pregnancy as the kits are calibrated ones, so unless the hormone levels in urine reach a certain level, they do not show positive results. If you suspect chances of pregnancy, you should consider visiting your gynecologist for thorough examination (to confirm pregnancy related changes/signs in body) and blood/serum HCG level test (to confirm/rule out pregnancy accurately). Blood HCG levels can show accurate results as early as about 11 days post-conception. Take adequate rest. Drink plenty of water. Maintain healthy diet. Be in regular monitoring and follow-up with your treating doctor/gynecologist and report any unusual/new/abnormal symptoms immediately.
Take care.