Sigh of relief, then more worry

I had a missed miscarriage last March. The baby stopped growing at 7w3d. 
Today we had an ultrasound at 8w2d. We saw the heartbeat and we were so relieved and happy!
BUT the doctor says he is dating the pregnancy at 6w6d. That puts us 10 days behind where we thought we were, and before when the last baby died. Now I'm nervous all over again that the baby isn't growing properly. I just want to be happy, but I'm nearly in tears worried. 
Has anyone else been here? Any advice?