3rd miscarriage but strange

I was expecting my period on the 14th. We have been planning to become pregnant after 2 previous miscarriages and so on the 11th I took a pregnancy test and it was clearly negative. That night I started bleeding which I related to my period. 4 days later I stopped but had some spotting that was unusual for me for the next 3 days so the following monday 18th I took a pregnancy test and it was positive. I knew I had miscarried. Ultrasound showed no signs of pregnancy. I would have been just 4 weeks on the 18th. My hcg levels were 73. My question is how was my test negative but a week after my miscarriage it was positive considering my levels should have went down but to have a pos test they would have had to go up....right??? So confused!!!!