Toddler Nail Biting

She (3yrs) has been biting her nails for a long time now and I can't get her to stop. However, I'm getting worried because before she would bite nails she would pull her hair as a baby. If her nails are too short to bite she'll try to bite her toe nails (gross I know, but I always tell her to stop). AND when we tell her to stop biting nails, she starts pinching and pulling her bottom lip. Doctor said it's fine as long as she isn't biting nails to the point they are bleeding, but I just feel something is up. Advice?