I feel like a b*tch

So today my boyfriend, his sister, and I were out and about. Me and his sister were playing and joking around about him (so I thought). But his sister was saying some stuff about him and I was laughing and joking along. So later on today my boyfriend said that she was speaking negatively about him and I was completely wrong for what I did by laughing and joking along with her and agreeing. He said that he will never do that to me and it made him feel alone and I should uplift him but not agreeing with her. I told him I was sorry and I won’t do it again. But I didn’t know I was coming off that way. I was just playing. Now o feel so bad...