Hello,
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Sometimes urine pregnancy test can show negative results even though you are pregnant. This happens due less concentration of pregnancy hormone (HCG) in urine.
I would recommend you to get examination done by your gynecologist to see if the abnormal bleeding that you are having is due to pregnancy or any other hormonal imbalance.
You will need to get a blood beta HCG test done to confirm/rule out pregnancy.
Take care.