Miscarriage confusion??

Last week, I had some bleeding with working out, so my dr got me in that afternoon. I should have been 7 weeks 1 day, but the sonogram showed 5 weeks 5 days with no fetal pole. My dr confirmed dates with me, and I was told I had a missed miscarriage and was sent home to see if my body would process it naturally. I spotted all week (no bright red blood or clots) and seem to have lost pregnancy symptoms. I did take pregnancy tests twice during the week, including this morning and am still getting super dark lines. Today, I went for my follow up appointment where we were preparing to schedule my d&c. I had my second sonogram and the baby now measures 6 weeks 1 day and has a heartbeat! I am shocked and numb from the roller coaster of emotions from the week. My dr says she literally has so explanation of how my dates could be measuring so far off (since I tracked ovulation) but she’s ‘cautiously optimistic’. Baby’s heart rate is low, which could be ‘normal’ for a baby measuring only 6 weeks. I go back next week to see if the heartbeat goes to to 110+ and baby continues to grow. I’m terrified to get my hopes up and my husband and I are still so shocked. Has anyone had any similar experiences with measuring that far behind? What was the outcome? Am I setting myself up for heartbreak again?