Thanks for the replies, all.
Maybe it's MSG. I looked it up and I could only find this one post from a Chipotle employee saying they don't use MSG (
https://www.facebook.com/chipotle/posts/425943579252). Elsewhere I've looked on the internet, people say they've directly emailed Chipotle asking about MSG use and that Chipotle doesn't give them a straight answer, so it's not clear to me whether they actually use it or not.
I didn't overeat (I'd eat half or a third of the burrito) and I always got it takeout.