Hi,
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Your symptoms (bloating, cravings, headaches, fatigue, nausea, heartburn, constipation and really really sensitive nipples, darker colour around the nipple, hard stomach, missed last 2 periods) indicate pregnancy. Sometimes the urine pregnancy tests can show negative results even though you are pregnant (false negative). This happens due less concentration of pregnancy hormone (HCG) in urine during early pregnancy. Visit your gynecologist/healthcare provider for an examination, blood HCG levels and an ultrasound scan to confirm the pregnancy.
Take care.