Doctor office claims I'm not pregnant!!!

I took four tests yes four pregnancy tests from 5/23 to 6/2 all said positive then on that Wednesday after 6/2 had some brown spotting nothing unusual to me then that Saturday I had a blood clot come out when I went to the bathroom had no signs or symptoms of a miscarriage got blood work done that Monday had an appointment scheduled for that Thursday 6/16 I believe. Doctor office called and said your blood level was a 1 you are not pregnant. I was in complete shock like how could this happen? Doctor office said they didn't know why my level was a negative if I was in fact pregnant with all those urine tests. They still don't have any reason on why my level was a 1 for a non pregnant person. Any advice ladies it'll put me at ease