Or perhaps you moved to a professional environment where people are on average much better at math than the average person.

It is not uncommon to hear objectively bright and hard working young people wonder if they have become dumber or if they have been a fraud the entire time, after they leave their high school where they enjoyed being a star student and move to a nice university where they compete with the brightest mind of the entire world. They are not dumb, just not mentally adjusted to an environment where they don't get to be the number one no matter how hard they try.