Bfp after missed period, confused

[Daisydays1]

Last period was 5 December, 3 weeks later (a week earlier than af was due) had brown spotting for 4 days tested after this tested bfn.
Two weeks later had a HCG blood test to check levels,bfn on blood test.
2 weeks later tested on two tests when next near AF was now due BFP 2-3 weeks on clear blue. I am very confused

So I am now confused when I could of conceived 🤯

I guess sometime near or after you had your bloods, as those will pick up even small amounts of HCG.