A false positive pregnancy test?

So my period was suppose to come sat and it never did. So on monday I took a home pregnancy test and it came out positive. Today (tuesday) I went to the dr to have a confirmed pregnancy test and before I left the house I started bleeding. By the time I got to the dr it was getting pretty heavy. I thought I was having another miscarry. So they had me pee in a cup and that pregnancy test came back negative. Then they did a blood draw and that too came back negative. She said I was never pregnant (so I didn't miscarry again). What would cause my hcg levels to go up one day and not be there the next day? I am not on any meds, I don't smoke or drink. Could stress really make my Hcg levels to rise?