first ultrasound and worried..

so I'm supposed to be 8 weeks exactly today, based off of my "period" however my period was a chemical pregnancy miscarriage.. they say to still count it as the first day of your period. so I get my ultrasound today and the baby is measuring 6 weeks 3 days.. has anyone else experienced this and all turned out well? I go for a repeat in 2 weeks.. it's possible that we had a later implantation right? My doctor didnt seem too concerned but a little. I need help.. lol. also he couldn't find the heartbeat but said that he wasn't expecting to yet...