I just checked in for my flight (economy fare purchased this May) tomorrow and on the check in screen where it asks you to choose number of bags, it had the expected fees listed for checked bags on the left, but on the right side of the screen, it had options of 0 carry-on or 1 carry-on, but the 1 carry-on option was free. I clicked through and finished checking in. My boarding pass says "carry-on" on it.

Does this mean I can bring a carry-on without being charged a fee? Any idea why it was free?