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It is difficult to comment on the cause without examining you. Your symptoms could be suggestive of possibility of pregnancy. Urine pregnancy tests can show false negative results at times hence they cannot be relied upon for accuracy. You should consider visiting your doctor/gynecologist for thorough examination and further investigations to look for the cause (infection, inflammation, ovarian cysts, PID etc) and get started on treatment as indicated. If you suspect to be pregnant, consider getting further examination done (to confirm pregnancy related changes/signs in body) along with blood/serum HCG level test and ultrasound scan (to confirm/rule out pregnancy accurately). Take adequate rest. Drink plenty of water. Maintain healthy diet.
Take care.