This happened to me too. I went to college with a high enough SAT score that I skipped straight to Calculus 1. I took Cal 2 and 3 and Differential Equations and managed to pass them all despite very little comprehension of the material. These classes were my lowest grades overall. I never took precalc or trigonometry so I missed the foundational concepts. Right before I graduated, I needed a few hours to round out my math minor, and I took Trigonometry, and all of Calculus suddenly made sense. I really wish I had not been allowed to skip Trig or Pre Cal!