What caused false Negative pregnancy tests?

I took a pregnancy test about five weeks ago, because the IUD I was using for birth control decided it didn't want to be inside me anymore. Anyways the test was negative, I then had some massive bleeding that lasted for about two weeks. I went to the ER a few nights ago with flu symptom was given a pregnancy... show more I took a pregnancy test about five weeks ago, because the IUD I was using for birth control decided it didn't want to be inside me anymore. Anyways the test was negative, I then had some massive bleeding that lasted for about two weeks. I went to the ER a few nights ago with flu symptom was given a pregnancy test which came back positive, then an ultrasound and was told I was much further along then 5 weeks. The baby in the image was definitely fully formed and I began feeling kicking today, so based on comparison to pictures online I'm assuming I'm at least 12 weeks along, if not further. What would cause the false negative previously? Real causes nothing disrespectful.
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