Strange spotting

Okay ladies,

So I had a period as normal June the 17th till the 19th. My cycle is 28days normally, I was then due to have my next period on the 17th of July but I didn’t have one, nine days later I started lightly spotting, only a pink very very light blood and it was only there when I wiped, not enough for a pad and certainly not a tampon. I’ve done pregnancy test after test and all negative. I had a blood test taken the morning before I started lightly spotting and that also came back negative.... any ideas what’s going on? Could it have been implantation bleeding and if it was would it have shown up on the blood test? Thank you lovely ladies 😘