My 12 year old daughter has developed brown spots on her neck down from her chin. Looks like birth mark or pigment change - this type of coloration - they kind of run together. When it first appeared I thought her neck was dirty. I think more developed after she was in the sun - althought she did have on sunscreen. What could it possibly be? She does have a little bit of it at hairline on side of neck too. She is very fair skinned, small for her age and thin. She is approaching puberity. Thank you.