It's just a hotspot, very common with BFS, they are scary but don't let the sudden appearance or reloacation of a twitching area scare you.
Often it has been my experience and the experience of many others on this board these things are temporary lasting from a few minutes to a few weeks sometimes months. Many of us have a seemingly permanent spot or two also.
You sound as if you may have a fixation on the term 'progressive' which you've probably catologed when your heard it used to refer to the progressive muscle wasting and atrophy of MND sufferors. Now it is a standard term which you use to measure your own symptoms and unconsciously try to align yourself to. We all do this in the beginning, and sometimes on ocassion even after having BFS a long time, but it's just part of the normal BFS thing.
Keep in mind, changing, spreading, hotspots, and a few lingering spots are still very consistent with BFS.
Search on the word 'hotspot', you'll see what I mean.
-dranyab