Chemical pregnancy or postpartum hcg traces?

I’m almost 8 weeks postpartum. Had unprotected sex just over 2 weeks ago. I had 2 positive strip tests (cheap ones from Amazon) yesterday. Today I took a couple more tests - a cheap one and a clearblue early and both were negative. So I went to the lab to get a blood test to put my mind at ease whether I’m pregnant or not and just got the results and I’m definitely not pregnant. But I am confused how the 2 tests showed positive yesterday as the lines came up straight away and they were faint but visible. Darker than the first positive test I had with my son. Is it possible this was a very early chemical pregnancy? Do the hcg levels drop just like that? Or is it more likely some traces of hcg were left from pregnancy? I read they can stay up to 8 weeks but that’s uncommon. I would appreciate if you can share your thoughts or if you maybe experienced something similar. Thanks!