Bleeding but not sure if it's an early period or implant bleeding

My so and I had sex on 9/22 and I started to bleed during (it happens sometimes) and shortly after that happens it usually stops bleeding the latest by the next day. But this time it went from new blood (red) to old blood (brown) and is still bleeding brownish red three days later. What do you think is causing this? FYI af isn't supposed to show until 8/1.