What airline is this? I looked up some AA fares.

What I found is that a Q fare is available even at the last minute for $440+tax roundtrip (pretty cheap for a non-stop at the last minute on AA!) This is non-refundable and has a $100 change fee.

I also found a Y fare for $876 one way. I didn't look for other booking classes, such as B.

So, if your ticket was around $1000, it might have been one half of a Q roundtrip plus a one way Y (220+876=1096). If the outbound trip was Y and the return trip was Q, then changing the return flight would be subject to Q rules, and that would result in a $100 fee. If the return trip is Y, then you could change the return trip with no fee.