Confused!

I had an ultrasound yesterday and was measured at around five weeks- was meant to be about 6 1/2 weeks... So i was instantly disheartened and then last night i began getting cramps and started bleeding so i thought straight out that i have had a miscarriage. Was absolutely devastated, but then today i had my doc appointment and he had my blood test results (which i thought would show a low level pregnancy hormone) he said that im in the 2000s and not to worry just yet til we have more bloods done... He knows about the cramps and bleeding yet thinks there may be hope yet.. I don't know what to think anymore eeeek