Middle school (well, Junior High) was the first time I couldn't understand a concept the first time it was explained. I'd had almost no experience of that K-6 so I had no idea what to do and thought something had gone wrong with my brain. Seriously. It was especially bad in math where "understanding concepts" plus some memorization is basically the whole deal, at least for most of K-12 education. I'd gone from among the best at math (with zero effort, because I probably should have been moving through the curriculum 2-3x as fast as I had been) to barely holding on somewhere in the middle, in a matter of months.