Am I a rebound?

We've been together for a year now and we're doing good until we heard the news that his ex will attend the wedding of my bf's brother. My bf got all moody since the news and keeps telling me how he hates it that the ex is attending. Now he's watching a movie about a guy who can't move on with his ex and even took a screenshot of the movie and posted on facebook with a caption "i shouldn't be watching this".

Should i start to worry that i may be a rebound? Or i'm just overthinking?

PS. It was after 3months of their break up when we met and started going out.