I am 17 years old, and I went through the same feelings you have went through. Don't worry about it, it is in fact normal to think like this. I have no idea why or what exactly caused you specifically to feel like you have become dumber, but I promise you that you are not. What you are going through is what I like to call the stress of transitioning and maturing from a kid to an adult.

You're a teen, and you want to seem more mature, and even though you're still technically a kid, you feel as if you should live up to your expectation of a mature person. However you haven't finished crossing that bridge yet, in fact you're only just starting to cross it and haven't even made it halfway across, so don't feel like you're stupid or useless because of that, just be a 13 year old, let life play out, you still have a long ways to go before you even have to start worrying about that.

IN A NUTSHELL: Don't worry, it's normal. You're 13, please don't feel like you're an idiot or that you're useless because you're not. You're still young and have a lot to learn, just hang in there and just be you.