I have heard that from other people. There two options as I see it. 1. You thought you saw the curvature, as your brain is so used to the idea of the ball earth. 2. You where looking though a curved window, that distorted your view, so that there appeared to be a curve, similar to when people use a fish eye linse to record the earth.

Most likely its number 1.

I recommend seeing Eric Dubays, Odd and FEA (Flat Earth Asshole) on youtube... there are many ways to see it, it might take some time to deprogram your brains.