Negative test turned positive!!

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I don'tknow where to start so I will just start writing. A year ago I fell pregnant. I took a pregnancy test but it was negative so I carried on with my life. Two or three days later I took another test because I felt weird and a new test was positive. So I dug up the old test from my bathroom disposable bin and the old test was suddenly also showing a positive. I was pregnant!!!. Now, I have done lots of research, but nothing conclusive on any medical papers or google. Has anyone else experienced this???