I was similar except it happened a bit earlier. In the UK the last compulsory school exam, GCSE, seemed like a memory test to me. I could often just figure out stuff in the actual exam even if I'd only briefly overheard something mentioned in a lesson. I didn't do a single piece of homework once I realised there were no real consequences and messed around in lessons. Got top marks in all the subjects I cared about and median in everything else (like art, where I didn't even bother to hang up my final piece).

A-level is like the stepping stone to university. It's optional and you only do subjects you like. Suddenly you had to actually understand the stuff. It wasn't just a memory test. The first exams I got B all around. I wasn't happy with this so I started doing homework etc and actually understanding things.

Coincidentally, my breakthrough also came with Newtownian mechanics. I was quite good at integration, but I didn't understand why I was doing it. Suddenly the whole thing started to make sense when I realised it was about rates of change etc.