My transfer was on a Friday. I had some very slight spotting that following Tuesday evening. It was only when I wiped and lasted for about 2 hours. I assume that must have been implantation bleeding, but it's later than I thought it would be.
On Wednesday, I really started to suspect that I was pregnant. I know this sounds weird, but I just smelled different, and I had a similar experience when I was pregnant with my son. I can't really describe it - it wasn't a bad smell, but I don't normally have any smell in my cm. By Thursday evening when I got a shower, my skin smelled differently too. By Friday I was driving myself crazy, sure that an HPT wouldn't show anything yet, but couldn't wait anymore so I took it!
I'm really sensitive to scents, so I'm not sure if everyone might have this to some degree and just not notice it? Start sniffing!