Low HCG

So from my LMP I should be about 6 weeks and 3 days. My first quantitative hcg was drawn on 9/8 and it was 53.24. I had another drawn at my OB yesterday (9/13) which resulted today at 84. I'm terrified. It's rising but hasn't doubled. I've had light cramping here and there, but no bleeding or anything. This is my first pregnancy, so I have no idea what to expect. Does anybody have any insight? Thanks in advance.