When adding an adjective to a noun, if the noun must become definite, why was “shirt“ not definite in your first sentence? Okay, here’s my day-later edit to my comment. I would delete this comment, but others have kindly answered my misperception, so in order for their answers to make sense in the thread...radagast said that “when adding an adjective it must become definite” and of course I should have understood that meant “when adding an adjective, the adjective must definite”. However, I misunderstood him to mean “When adding an adjective, the noun must become definite.”
Second misunderstanding: I thought חולצתי was not definite b/c it had no -ה in front of it, but as YardenNB helpfully pointed out, a definite noun is not always preceded by -ה. So I finally realized that חולצתי is definite!