I just got my HCG result and am quite scared, they are only 324 from last Saturday when I was just on 5 weeks pregnant. It should be much higher

maybe someone can help me with calculations?

My last period was 8 Sept
We only had sex once that being 29 Sept so must have ovulated late
Got BFN on 16 Oct on HPT
Got BFP on 18 Oct HPT
Had HCG first done 19 Oct which was 64
Had 2nd HCG done on 23 Oct which was 324

Do you think these are ok? or not a good sign for the pregnancy. With my last 2 girls, my first reading was around 74 and 2nd was in 3000's so im scared.