Hcg testing low

So I went to the doctor yesterday and she did an ultrasound saw a very small sack I was supposed to be 6 weeks and 4 days pregnant she said what she saw was more consistent with 5 weeks so she did it HCG test to check my levels the results came back today at 650 She said either I ovulated late and not as far along as we though or I am having a miscarriage has anyone else had this happen and ended with positive results I go in tomorrow for more blood work hopefully the numbers double