The color could veer from bright red to darker red (sometimes close to brown) at the very beginning and very end of your cycle. That doesn’t mean anything’s wrong, Dr. Carcamo says. Rather, the dark color is likely just older blood that has spent a longer amount of time in your body. “It’s because you’re not bleeding as much, so it takes longer for it to fall out on your pad or your tampon,” Dr. Carcamo says. During that time, the blood oxidizes, which makes it darker.