1st App... Neg pregnancy test

Had my first app today - went in and my test was negative. Not pregnant. Which makes no sense after 3 positive tests up until a week ago. Did an internal ultrasound and there is what may be a potential sac - just very very small so really hard to say for sure. The doctor couldn't confirm pregnancy with it because it could have also been mucus or whatever.  Did blood test and I will get the results of that tomorrow... If it's + then I go back in Monday for another test to see if my HCG levels are rising or falling. If it's neg - well then I guess I have 2 mcs under my belt. Feeling so depressed and frustrated right now.  Has this ever happened to anyone?? I thought I was 6 weeks and to walk away with nothing is just so disheartening.