Hcg lab error??? Or what??

Amber • 28 yrs old-married for 8 years. Lovebug born 11/3/09 BFP after miscarriage and before AF. Due end of August
I went for an ultrasound and they saw fetal pole, yolk sac, crown to rump length is 3.6mm, heartbeat at 111bpm that puts me at exactly 6 weeks- which is right on going off lmp. But then they call and say by hcg level was 1651??this was all yesterday on 11/4.
My first hcg at 10/20 was 356 and 10/22 was 651. 
I've had no cramping or spotting at all. Seems strange that my hcg would be so low when my numbers were perfect earlier and everything on ultrasound is exactly what's it supposed to be... Thoughts??? Help please! I'm driving myself crazy!