My teeth moved in a straight manner very quickly after wearing braces. Is it normal?

Hello doctor,

I have a braces-related question. I wore the traditional metal braces on my upper jaw as a teen, 10 years ago. I wore the retainer for five years every night, and then I reduced it to one night per month after that, as instructed. My front four teeth shifted a lot (they are crowded, so it was a quite visible shift, some were pushed out, and some pushed back). The retainer could make a big difference in one night when I wore it, but I was advised to get a completely new brace. So a few days ago, I got a lingual brace on the front teeth (the social six). My concern now is the speed of shifting. A couple of hours after getting the braces, my teeth were significantly straighter (1 to 2 mm difference). A day after they look almost perfect, I was told I would be wearing the braces for six months to a year. How is such a drastic change possible so quickly, and is it healthy or normal? It hurts always, but it is not unbearable. And could I have used a new retainer instead of getting new braces then? I hope you can help with my worries.