DS13 is experiencing the same change. He used to be meticulous to a fault (slowed him down and he wouldn't move on to another problem until he figured the first one out.)
He advanced two grades in math and I think in an effort to keep up, he's making simple errors. So...what we've done is give him more practice and ask him to do out all of the steps on paper. When he'd do some or most of it in his head, there would be more room for mistakes. It's a matter of slowing him down, teaching him to go back and plug answers back in to see if they make sense.
Good luck, Ultramarina. I think it's easier for these kinds of mistakes to happen because math suddenly has so many more steps.
