I'm not an expert on women's' undergarments nor do I come across them everyday lol, so if it's a silly suggestion I'm sorry. But, maybe it's her bra?
It could very well be a tattoo, but we may never know as it would be in an area that is typically covered up. Maybe if she posts a pic of herself at the beach in a bikini or something like that, we could make out what it is.
If it
is a tattoo, honestly, oh well. I don't necessarily approve, but it's not like it's gonna be present in every picture from now on like the arm tats.
