The flashback appears to be a temporary excitation of your chronic anxiety. When suppressed feelings have built up, it can be expected that they will surface on their own schedule. We don't know exactly what might cause them to appear intensely for a day or two. The important thing is to apply the steps of the EC Process as they appear. Working with the breath can be very helpful, as you mention.
We usually think of anxiety as the first appearance of a deeper level, core feeling, which might most likely be fear-related. Try to see the core feeling behind the anxiety, and work with that, although you can also process the anxiety itself, and that will lead to the underlying core feeling.
The meds may be necessary at this point, but you should realize they inhibit your ability to see (and feel) the core feelings, so one of your first goals is to reduce the meds cautiously. You're going in the right direction.
