I know it's easier said than done but try not to overthink the situation. I often make myself miserable over thinking every little thing so I can relate to this concern! If she's aware you're quiet but you made the effort to wish her a nice Christmas also (instead of ignoring her and leaving), albeit in an unusual way by saying "yourself" instead of the normal "same to you" kind of thing she probably appreciated the sentiment but found the delivery unusually worded so I wouldn't say she was laughing at you - just the unexpected response.

And worst case scenario if it was directed at you who cares what she thinks lol. You do you, pal. Have a nice Christmas!