When does HCG actually start to show up?

I got a bfp on Saturday and had what I think was implantation spotting Saturday night (very faint and didn't last long), would my hcg levels begin rising from the point of the implantation bleed? It's been three days since the bfp and spotting, but my hcg was only 31 today. Normal? Or does hcg begin from the day of conception? Trying to have hope that it's ok that my levels are low. ☹️