This happens to me!! Or at least has happened. I moved from Portugal to Brazil a few years ago and back in Portugal I was speaking very fluently and speech therapy was actually working. When I moved, I spent about two months almost not talking at all. We were in vacation so I didn‘t have classes and because I knew no one I barely talked. I barely remembered I had a stutter on my first day of school. Turns out I had a horrible relapse and it took me sooo long to get back to the place I was when I left Portugal.