Low HCG levels at 6 weeks

I'm currently 6 weeks pregnant and I've had to do some blood work lately. . My levels are coming back low (not a normal range) but did double in 4 days time... I've had no bleeding or cramping...

Has anyone ever experienced this? And did u go on to carry full term?

Dr says not to worry right now... waiting for more blood work to come back on Monday. .. but... it's really hard not to worry!!!