Orange stains/ cotton damage - on black cotton underwear

[Sam2112]

Sorry for topic, but a mystery.
I have found that i have orange stains on my black cotton underwear. In the gusset area.
Yes i have experienced heavy periods, and excessive bleeding, but it is NOT blood stain, nor "discharge". The mark is as if the cotton had burnt.
AND only one black cotton. The actual cotton is marked.
ANY IDEAS?

not just one brand -

Yep, normal. I went to a comedy show last year and a female comedian made a crack about women's comfy knickers with the beetroot red gussets and all of us women in-the audience nodded our heads!

excellent - even as intersex person, living as male, I understand the need for decent and comfy pants - so many ruined by being a leaky human -
One has to see funny side -

I only ever developed this with the copper coil. As soon as I had it removed, it stopped.

interesting .. makes you wonder - I have had coil before and don't think i had such issue back then -