How common is it?

The last few months AF has come on day 32. I've had negative tests on day 33 and 37. Tomorrow is day 40, and still no AF. I had extremely sore breasts and extremely light spotting from day 21-26, which looked an awful lot like implantation bleeding. Another Glow user suggested I could have a pregnancy which isn't detected on a HPT, so I looked it up and found a nursing board with all these nurses talking about all the patients they've seen with this issue, some not testing positive until 8 weeks or so, except with a blood test. Is this more common than I realize? I've had three pregnancies and never experienced this.

Thanks ladies!