So I truly don't see how an INFJ could become nhilistc unless something REALLY traumatic way beyond what I have experienced happened.
Mh... I'd say idealists are always looking for meaning, especially introverted ones, and they can suffer when their beliefs are shattered for example and they realise their conception of life was just their conception and not a universal principle.
Nihilism and not caring are very different. Nihilism is emptiness. You cannot care because nothing seems to matter ultimately. And this void is scary, because you are a human and you need a drive to your life to keep going.
I'd say INFJs would be prone to nihilism because of their ability to see things from a lot of different angles, resulting in a lack of definitiveness in their perception of the world in its globality. INFPs for example would have very strong beliefs they go for, and would go through a very hard time when something shatters them, but INFJs are less likely to take such a strong position, and so might end up feeling like they cannot just force a meaning on their life, even though they long for one.
It's funny, I have talked to my boyfriend about that a lot. He's an INTP and the thought that life is meaningless is actually reassuring to him (kind of like the feeling you wanted to get this summer by not caring about failure anymore), whereas to me it can be very depressing.
It's a matter of perception, like shai and satya say (great posts by the way).
As far as being a college thing, I'd say college might be a period in which you are more likely to experience that kind of thoughts because you are at a point in your life in which you are trying to project yourself in the future without really having any experience, and you're at a transition point from childhood to adulthood without really having any landmarks yet. I'd say they might be other moments of your life when you're likely to experience that kind of thinking too though. Midlife crisis type thing, or maybe the end of your life. Just speculating.