Sorry, but this child needs a diagnosis. Something is going on with her. I'd say she needs to see a doctor and probably a therapist but you might try to talk to her and ask her why she thinks this is a good idea? At 16 she is well aware that this isn't at all safe or sanitary and can spread disease to herself or others, and knows this just isn't appropriate, so she's acting out. Your job is to find out why and what it means for her, and that again might require a doctor and probably a psychologist. Peace.
This is a rough question, but ask her about trauma. Changes like this are sometimes associated with sexual trauma.
Hi, I am also on the ADHD community as the forum leader and this behavior would be kinda normal in that community. Basically, a child with either ADD or ADHD gets so involved in what they are doing that they resist the urge to go. Its only when that urge get so bad that they will make a break for the bathroom and many times don't make it. If your daughter is very intelligent the normal signs of adhd/add can be missed. I have known of kids that really didn't have any problems educationally until grad school. And saying that she likes it could be a cover. Any way, lots of info out there if you or she are interested. a starting place might be here.... https://adhdonline.com/women-with-adhd/?gclid=EAIaIQobChMI3PHz9O7C8QIVWyutBh0GCwTnEAEYASAAEgKcwfD_BwE