Bleeding and cramping ?

Finally got my BFP about two weeks ago! Best moment ever after trying for 5 months!! Then i had an ultrasound on tuesday and the sack only measured 5 weeks which is wrong cos my calculations said i should be at least 6 1/2 weeks. Then that night i started bleeding and cramping. Doc said my blood results came back with hcg in the 2000s so not to get upset just yet. How can he have hope! I have clearly lost the baby :( i mentioned the blood and cramps and he shrugged it off just saying that can happen in pregnancy as well.. Ill find out monday if my hcg is rising or lowering! But the cramps and amount of blood is telling me ive lost it. ? has this happened to anyone here?