early ultrasound - nerve wracking results

Mi
So, I went back to my OB after asking for prayer from y'all, and she was very pleased with my HCG and progesterone levels, as they were rising. She was a little concerned about the size of my sac. 
I went back for another ultrasound, and the sac is growing (praise God) and she detected a heartbeat (😀). But, she said she's a little concerned because it seems as if the implantation occurred reallllly low in my uterus 😩. And she can't explain why. So, she has me seeing a high risk pregnancy doctor. 
Have any of you ever experienced this or heard of this? Any insight?