I know a bit about this. I am the same type A personality when it comes to academics.
My grades and school in general really defines who I am
This statement worries me. This is how I was - and it was a trigger for a lot of panic. Not so much when I was studying, but after - when I did not have the constant cramming, test taking, and research. Because my body was used to a high stress state, whenever I had a break from school I would fall into some of the worst panic attacks. My body needed to be in its heightened state of anxiety to exist, it was what it was used to. Maybe you are going through some of this. Last year you took a full course load and you were in constant over-drive. Your body may be reacting to some of the let down of having a bit more free time and being able to slack.
You need to balance your academics, and not let them define you. I know how hard it is to get good grades, but getting straight As is not worth your health and your long term mental health. You need to figure out how to be happy and healthy - then whatever grades you can achieve with this landscape is exactly where you should be. I am not saying that you should slack off and get Fs - I am saying that you should prioritize health and happiness first. Work hard, but be balanced with it. If that means getting a B+ sometimes, then so what? You may find that once the self-imposed pressure is off of you, you will perform very well.
Am I becoming dumber?
No - but stress and high anxiety can impact memory and test taking quite a bit.
To give you a bit of my background, I just finished my doctorate last summer. I know the pressures, and I know what it is to define yourself this way. When I finished up, I had a huge mental breakdown with panic and HA, something that I am still going through. This came from years of high stress and a huge work load. Trust me on the academics - the sooner you achieve a balance, the better off you will be. Work on defining yourself in other ways. I know many brilliant academics - who have no life and are not happy people in general. They sacrificed everything to be brilliant. The smartest and most successful people in my institution are those who have a work-life balance. They may not publish as many papers or write as many grants, but they are happy and hard working and well-rounded.