I've never been to a game in Chicago but I can tell you what happens in Minnesota if you wear a Bears shirt.
I've been hit in the head with a snowball, seen Bears players get hit in the head with snowballs from non-existent security. I've been cornered in the bathroom and called an expletive for no reason other than the fact the Bears were destroying the Vikings.
I've been threatened on the light rail on the way home by some drunken idiot who I could have beatup with 1 hand tied behind my back. I asked him what his problem was, his response "If there is one thing I hate more than Packers fans, its Bears fans, my goal tonight is to kick the %##$ out of some Bear fans". Nevermind the fact I have always been with my Vikings buddy who has season tickets up there.
None of that even comes close to the scuffle I saw in the stands once when I went up with my friend to a random game, wasnt even Chicago. This guy had a "Favre" Packers jersey on, now mind you, this is when Favre actually played for the Vikings!! So you guys hate other teams so bad you decided to start fighting this guy even though he had a Favre jersey? This guy wasnt even doing anything, but all of a sudden gets beer poured on him and fans screaming obscenities at him.
You could make the case that this happens at all stadiums, but I've been to Kansas City with my dad numerous times because he is a Broncos fan and nobody has said a word to either of us at those games. Weird.
Those are all bad. Are you SURE it's not just you?