Just wanted to offer some reassurance. I was scoring in the high 240s-250 around 1-2 weeks before my exam but then got a 234 on NBME 16 three days before which sent my anxiety skyrocketing. That was within a week of scoring 250 on another NBME. Ended up with a 258 on the real thing.
I think I was just really tired of studying by the time I took that last NBME and was sort of just trying to get through it rather than putting in a lot of mental effort. Of course then I felt really terrible in those few days before the exam and it lead to trying to cram more minutiae which I don't suggest for anyone as it is already a stressful time. But who knows, it ended up working out in the end.