ADD usually works the opposite sometimes. It is there in childhood there are signs and it can go away as one grows older and you will grow out of it. But as far as it just appearing. You do no grow into it ADD. The signs are there at a young age and sometimes will go away as we age.

Sometimes ADD symptoms can be present due to our environment. We are the product of our environment and we live in a technologically advanced society that moves at a very rapid pace. Therefore people sometimes think they can be ADD because the symptoms present themselves. But sometimes it can be could be known as pseudo ADD.
Or if a class or lecture is boring and uninteresting it could cause loss of focus or interest.

Also you mention you were the top student. It could be causing your feeling of anxiety or depression and you do not feel if you do not have everything right you many not be the best in the class again. You have such a high pedestal now and are feeling the pressure and stress to perform again. That you have to shoot for the stars. So you are developing different coping skills to try and deal with the pressure. Something that takes you mind off it and gives you temporary relief.

It could be depression as was mentioned or anxiety. Maybe try talking to a school counselor see if they can help you out or maybe try to see a psychologist or psychiatrist to try and get to the root of the problem.

--Maus5321