Does anyone else ALWAYS spot for the 4-5 days leading up to their period?

I have an 11 day lp and always start spotting brown/bright red at 7-8 dpo. My last 46 cycles were like this (when I got off bc). I thought it was a progesterone issue but my progesterone is fine. I've conceived once (miscarried at 9w3d) and believe I had a chemical pregnancy last cycle. The 5 doctors I've seen have not been able to tell me why this happens. I've heard people say it's old blood from my last cycle, but can anyone explain this theory better? Thanks.