Your teacher probably sucks. I barely got a C in my HS AP calc class. The teacher wrote terrible tests where she constantly featured the exception to the rules she taught in class....the tests seemed to be more about seeing if you were on your guard, not if you learned anything.
I got a 4 on the AP test. She really felt like an idiot after that--I clearly learned SOMETHING if I did that well.
Took Calc I in college and easily got an A--the teacher I had was a decent teacher and wrote decent exams. In my school, Calc I was roughly half of AP Calc (they stop at antiderivatives)
I had to drop Calc II in college after getting a D on the first exam which was nothing more than a review on antidifferentiation and a little bit of integrals. No volume or any of the remotely challenging stuff, which I already know.
As usual, the teacher is crap and is one of the most poorly rated teachers in the entire faculty.
Retook Calc II with a competent teacher and, again, got an A.
So if I in my school AP Calc was equivalent to Calc I and Calc II, how did I get a 4 on the AP test, an A in Calc I, and an A in Calc II, yet got a C in AP Calc and D on the first exam in Calc II the first time?
Last time I checked, Calculus has not changed significantly in the last few centuries, let alone 1 year.
There's only one variable--the teachers. Getting grades is about playing the game, not about learning anything.