Dark spots on baby's tongue

I'm a first time mom so everything is new to me. My lo had a white tongue since she was about a week old, someone told me she had thrush. At her first Dr appt, the Dr said they didn't think it was anything but milk tongue. The white got worse and there was a small dark spot. so I took her back to the Dr. They gave her nystatin and said thrush can sometimes be dark but they still were a little hesitant because only her tongue was white and nothing else in her mouth. I used the nystatin for about 25 days because I read that it could take about 2 weeks for the thrush to go away and I sometimes wasn't able to apply it. It didn't go away or even look better, the coating on her tongue was getting worse darker and thinker. So I made another Dr. Appt. The Dr didn't know what it was and almost wanted to do a biopsy until they started picking at it. I never actually tried to get it off cause I read that thrush can sometimes bleed if wiped off. The Dr showed me what it was and said it looked like lint. I can't figure out why anything would stick to her tongue like that. I got home and got the rest of it off and it was definitely lint. I felt horrible! I only found a few post about lint online and it seems like it has something to do with breastfeeding and I am ebf. Does anyone have any experience with this or know anything about it? My nipples aren't linty when I feed her. I don't get it.