Hello! Please remember that my response is for information only, we are not establishing a therapeutic relationship.
We go through various developmental stages through out the life span, and each brings it's own "tasks" that we have to work through and come to resolution. As you know, for young adulthood, there's a lot of adjusting to new situations (finishing high school starting college or job) and new relationships. College can be a particular challenge for people with ADHD in that the structure is different and can bring new challenges. Also at this age, there's a push toward independence from parents, both emotionally and practically (and tension when that's not possible, which is often the case in our society today)--all of these things are stressful (even if they are positive changes) and can increase anxiety.
Here is a stress
check list that you can look at:
Typically each of these stressors is assigned "points" and the more points in that you have in a given year, the more likely one is to suffer consequences, such as anxiety.
To answer your question about if age/transition triggers the need for a change in medication, the answer is yes --sometimes people need a change in medication --although the exact reasons are not always known why one regimen works for awhile and then doesn't, it's why meds are monitored so closely. This is one reason that people taking medication need to have regular appointments with their Psychiatrist. A change in medication could help with her anxiety.
You can also consider some therapy --Cognitive Behavioral Therapy (CBT) is particularly helpful for learning how to lower anxiety and cope with ADHD. You can read about CBT here: www.nacbt.org/whatiscbt.htm
Whatever the causes of your daughter's increased anxiety --there is a solution, however, she needs to take steps to find the right solution --and of course this is where you can help her!