How To Confirm Pregnancy?

Recently I had a missed period and the day ebfore that I received a faint positive home pregnancy test. I waited until the day of the missed period to re test, again two faint positive tests. I went to the dr for blood work the day after my missed period and received notification that the test came back negative and that I was not pregnant. I waited the entire weekend and still did not have my period. ( I AM NEVER LATE ). So, I went to my old dr and they did bloodwork for me. That Dr. called and told me my level was jsut at a 25 and that I was indeed pregnant. I KNEW IT! However, of course the next day now what would have been 1 week after my missed period, I started having heavy cramping and it followed by extreme pains and back cramping. I ended up with what would seem to be my period and the next day passed a tissue that looked sort of cylindar like. I only bled for that day and had light bleeding for a few days following. I was certain that I had miscarried. I went to the Dr. today and they said my levels came back -2 and she doesn't think I was ever pregnant. Why would my levels have been a 25 with the other dr and had all the signs of pregnancy followed by miscarriage ... what could have happened??