BFP but terrified

Katie
My last cycle started April 12th.  On the 21st day I went for some fertility testing blood work since we have been trying for over two years.  They told me my progesterone was low and they wanted to put me on medicine.  On cycle day 28, the day of my appointment, I got a very light positive pregnancy test.  The doctor decided to change my appointment type and sent me for blood work.  They called the next day and told me "your numbers aren't high enough for what we would consider a viable pregnancy.  False positives do happen occasionally".  I was devestated.  A few days later I went back and had my initial "low progesterone" appointment where she told me again I wasn't pregnant (and that I probably didn't ovulate since my progesterone was so low at cd21) and that I should start soon.  Well... I still had not started as of today (5/22)
so I took a pregnancy test!  It's definitely positive.  The first one is fmu and the second was about 2 pm.  I want to be excited but I'm terrified.  How could they be so wrong?  I'm definitely going to call them tomorrow and insist on more blood work.