false positives??

Hi All, my husband and I are trying to conceive our second child naturally, I'm 36 and have PCOS so it's not so easy for me, we fell pregnant 6 months after trying and miscarried, a year and a half later I had pregnancy symptoms and took two tests which both came out positive. I had not yet gone to the doctor when I started bleeding at 5 weeks, went in and had another pregnancy test which was negative and also an ultrasound which gives no indication a pregnancy existed! doctor says it's weird as I should still have hcg levels in the test so was wondering if this has happened to anyone else? I'm positive I was pregnant but how can a test come back negative the day i miscarry?