I was in the same place last semester. I usually get 95+ but with Lin Algebra, I was getting 70-80's. The class average was abysmal too. I finished the class with an A and a 92% on the final. It just takes time to gain the necessary intuition. Also, yes, calc is supposed to be much easier. I was doing calc 3 at the time and it was nowhere even near 1/3 the difficulty of lin algebra (or so I thought at the time; looking back, the subject matter looks pretty easy and trivial- chalk it up to hindsight I suppose).