Ultrasound perfect but low HCG


Ultrasound measured perfect at 6w6d with a heart rate of 139. Pelvic exam was good. BUT my HCG is lower and following into the 4-5 week mark... what does this mean? I've had two previous MC's n I'm terrified... should I just concentrate on the Great ultrasound? I wasn't supposed to have a U/S until the 26th but they told me I had to call my doctor on Monday and come in for another test to check the levels... has this happened to anyone ?