Something strange has happened

Sooo... something very strange has happened I have 2 children right now on the 29th took 2 home pregnancy tests that both came up positive faintly but definitely could see the lines now I’m high risk so my ob told me to go to hospital for levels and us went to hospital they also confirmed through blood I’m about 4 weeks along and to follow up in 3 days so today I went back but to a different hospital closer to home and they said how did I know I was pregnant because my tests came back negative! I’m in complete shock how is this possible I asked dr if it’s possibly a miscarriage and she said it’s not possible because I should still have HCG in my body but the test is saying I have none! Now I have all symptoms of pregnancy including no period has this happened to anyone I’m so confused