-.01 hcg level?

I'm not sure where to post this but I was just wondering about something. On March 24 I got a blood test to check HCG levels since I was 12 days late with no positive pt. I finally received a pretty positive pt on the April 2. My nurse called me yesterday and I had asked her about my original blood test levels on the 24th. She told me it actually came back less than 0 at -. 01. How is that possible? And then to get a positive 9 days later? I'm so sorry if this is the wrong group😣