This is the kind of thing I've mentioned before. It's one of my pet peeves: the woman who's busty, dresses to accentuate her chest, but if you NOTICE, she gets morbidly touchy about it. YOU didn't do anything wrong. SHE mentioned her sorority, and you looked. What did she think you were going to do? And why, if she didn't want people noticing her chest, was she wearing a t-shirt with writing on it? The woman you mentioned can hardly get indignant if you look at the writing on her shirt--she's the one who put it on. (OTOH, it is possible she forgot there were Greek letters on her shirt.) I suspect you're not the first one who's gotten this reaction from her during an innocent flirtation. All she's really doing is convincing herself that all men are pigs with one thing on their minds. She's made up her mind, and so every ambiguous thing a man does around her gets interpreted in just one way. You are better off without her. If it hadn't happened then, it would've happened in your next conversation, or even worse, on your first date. Naturally, she stalks off in great indignation, leaving you to assume you're just a pig. The thought she might be overly-touchy or sensitive or even paranoid, of course, never crosses her mind.
Twice, in college (once with two different girls), I had a similar thing happen to me. Once I was playing pool, the other I was sitting in the dining hall. Both times I was with an EXTREMELY busty girl, both were dressed to accentuate their bustlines. Both times the woman made a somewhat lighthearted comment about her enormous bust. In both cases I followed her lead and assumed (apparently wrongly) that she wasn't self conscious, and made a similar comment. Both times, the woman immediately had the NERVE to get indignant. They both turned severely cool toward me from then on. I assure you, my comments were not obscene or leering, or even particularly specific to them. The reaction was totally out of place, like a Twilight Zone episode. In retrospect I think it's good that they both revealed what kind of people they were before I got too involved with either of them.
No, it's really a form of game-playing, and it indicates a kind of either insecurity, self-consciousness, or perhaps even downright maliciousness in the woman involved. If a woman dresses to accentuate her bustline, she has no right to get indignant if you notice. It just goes to prove that just having large breasts doesn't make a woman attractive--it's the personality that goes along with them, and sometimes they're a bad combination. Don't worry, you weren't wrong--she was. It happens.