When will my hCG levels go back to 0

My miscarriage happened 6 weeks ago when I was 10 weeks pregnant. The doctor said the pregnancy test they took at the end was 'almost negative' then I took one a few days later and it was definitely negative. Now I'm half way through my first cycle back (CD14) and I'm getting very faint positives?! Can someone explain to me how I could be pregnant when my first period back was only two weeks ago...