Well, I don't think your personality type can change without a traumatic event. What most likely happened was that, once exposed in the real world, your Te developed to a greater extent. You were also likely more comfortable post-highschool. Most NTs are.
What you're probably trying to say is that you were shy, and now you're not. Introversion by Jungian standards does not denote shy. You could have a Jungian Extrovert who's shy, or a Jungian Introvert who's not shy. Introversion and Extroversion on the Jungian scale, denote (in the simplest of terms), that an Extrovert is recharged by socialization, and an Introvert is drained by socialization. Though, you must note the caveat that even Extroverts are drained by prolonged socialization. I, as an ENTP (same goes for ENTJs, I would think), am more quickly drained by socialization than my ESFJ mother. I have a much higher tolerance to peeplze than my INTP uncle, INTJ father, ISTJ friend, INFP girlfriend, or all the other introverts I know. ENTPs are notably the most Introverted Extrovert. ENTPs, being NTs, do require extended periods of introspection to sort things in our nogginz. Same for ENTJs probably.
So you're either an INTJ who has matured and practiced an ability to socialize actively for a longer period of time (as my father has for entertaining people for work), or you're an ENTJ who was uncomfortable in highschool, and was shy or anti-social, and is no longer limited by that.
Alot of ENTx's will seem to be Introverts at times. Anywho, fun stuff.
-Analytical Psychology and Assesments nerd, what every High School Junior should do in their free time.
Goodies on my personality type:
Temperament- 67% Sanguine
MBTI- ENTP
Enneagram- 7w8, 3w4, 9w8. Tritype 783. sx/so
Socionics- ILE (Inventor)
Big 5- Extroversion
68%Orderliness
18%-Emotional Stability
84%-Accommodation
14%-Inquisitiveness
82%
Sloan- Global 5: sloan
SCUEI; sloan+ SCUe|I|; primary
Inquisitive; S(68%)C(80%)U(80%)E(78%)I(82%)