But I've seen it in myself at other times over the last decades, too.
(I didn't know introspection was harder for introverted? Reference?)
It's just that introversion is a personality trait whose meaning was overloaded by pop-culture and pop-psychology. Stuff by Freud, Jung, Eyesenck, Myers-Briggs, etc. is useful to know as much as it is useful to know about the work done at Xerox in the 70s and 80s. Informative and a foundation for later work, but by no means current.
This http://en.wikipedia.org/wiki/Myers-Briggs_Type_Indicator#Rel...
will tell you about just how inaccurate it is to take a classic test like MBTI. While there is something to be said about personality traits and nature vs nurture, Psychological Types are so 1923.
There was a lot more concrete research done in the last 20 years on depression and related mood disorders as it is somewhat of a modern-day plague (OK plague is too harsh, can someone help me with a disease analogy here?).
So to make a determination over why you are the way you are based on how you perceive yourself in terms of a personality trait category that (Jung or not) is related to the Psychology of Types is doing a bit of that self-fulfilling prophecy.
There's nothing wrong with preferring to keep to yourself and socialize less frequently, but perhaps it would be wise to rule out other hypotheses (like, really really convince yourself that there's not something else bothering you in life). That's as far as I can speculate, because although I might claim you might not be consciously aware of the whole story, I am even less aware, it's just a pattern that I noticed with some friends and some of them, years later, refer to those periods as periods of depression, some of them even talked to a good psychologist about it.
It's hard to self-diagnose with introversion, you might just be seriously depressed and not willing to admit it to yourself, but I don't know you at all so all this is maybe BS.