I don't know what is this. I noticed this months ago. All the panties/briefs have brown or black coloured spots or stains on the back side or front side or both sides. Mostly it's on the back side. After multiple washes also, they don't fade or go away. I am unable to understand what are these spots or stains. Clothing material is pure cotton. I have attached images for reference. • They are not anywhere near the vaginal area and neither it's bleaching or stain due to discharge. • These are also not blood stains. Thank you