Why after having corrective jaw surgery (upper and lower) would your jaw move again?

After having braces (with bands) for 3 years I was finally given the go ahead to have corrective jaw surgery. I had quite a severe overbite. both my upper and lower jaws were broken and a metal plate added to my lower jaw. Now after 7 years I have begun to have jaw pain all over again. I went to my dentist, who confirmed that my jaw has moved, and by quite a bit (with my teeth closed, I can fit a good portion tongue out the gap between!) . Has anyone ever heard of this happening , and reasons why?

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