HCG rising but not doubling! Help!

I know this has been posted before but I'm just looking for advice or support. I had blood drawn at 4w5d on Wednesday and it was 670, retested at 5 weeks today and it only went up to 900. Has anyone had anything similar and went on to have a viable pregnancy? My OB is having me come back in on Monday to check progress, but I'm freaking out and so scared.