I'm having a ball
●Don't stop me now／I'm having such a good time／I'm having a ball という流れで歌われる通り、"have a ball" = "have a good time" という意味。
●英英辞書では "to enjoy yourself very much" "to have a very enjoyable/exciting time" など
○"So, how was the party last night?" "Oh, it was great! we had a ball!"
○It was a great trip. I had a ball!
○I thought I'd be travelling the world by now, having a ball.
○Everyone had a ball at the party.