You got the feeling from Carol that bringing something would have been appreciated? You did. You brought Carol. If she thought they expected a contribution to the party, or a hostess gift, that was her responsibility. You were only there because you were doing her a favor. She should have been going out of her way to make sure you were okay, and covering for you with anybody who needed it.
A thank you note would be nice, since you did get to participate in more of the party than you thought you would, but anything beyond that is unnecessary. Don’t sweat it.