why did i think erin was a cop?

because she was an agent provocateur. i caught her trying to incite violence several times, and she once tried to talk me into smashing up somebody's car (i didn't). it was classic, textbook undercover cop shit.

she was also way too old to be a first-year student.

there were other tip-offs. but, the incitement/entrapment is the classic giveaway, and she was about as obvious as could possibly be about it.