You are about to embark on the biggest emotional transition of your life which is leaving home to start your adult life in college. You are giving yourself time to think but you are not dealing directly with the underlying anxiety. It would be useful for you to talk to someone about what you are feeling and sort out your fears.
I don't think it was an external trigger, but an internal one because nothing around me changed.
Also I have no reason to be stressed, anxious and/or depressed. I have 3 months of relaxing with at least two holidays. No work or school or commitments. Then I have the best university offer I could ever have hoped for. So everything in my world would be rosy, were it not for having to be around people.
I would say, yes, there probably is something wrong.
I wonder what triggered this change in behavior?
Not being able to tolerate other people puts you at a huge disadvantage. In life in general but especially when you go to university.
Isolating oneself can be a sign of depression. Disorganized thoughts can be due to anxiety or depression or some other mental disorder. Or be from stress or feeling overwhelmed.
I would advise talking to someone (a doctor, counselor, psychotherapist, etc). I would have the problem evaluated and then work towards trying to resolve it. It's also easier to resolve these issues when we're younger and they're not so entrenched.
J