You said your parents are very happy with your 3.5 GPA at Purdue. You should be happy with that.
I think your parents are probably concerned about your goal and your future progress in college. Did you volunteer to take Multivariable Caculus class or did your parents suggest you to take it? If you decided to take it in order to be ahead in your college plan then you should have received credit for it although the class was hard and you were busy with your intership. If you set the goal then you should aim to achieve it. Probably that’s what your parents have in their mind. (I took a summer class in assembly language but I dropped it too because I was afraid I could not get at least a B, the minimum grade to be admitted to CS major. My parents did not know anything about it though.).