Help! What does it mean if my boyfriend was rubbing another girls back?

From time to time I think about this and it makes me uneasy.

It happened at least 6 months ago but me and my boyfriend were hanging out with some of his friends (he's basically known them his whole life, it's a small town) they all went to school together. I don't know them that well.

My boyfriend was standing with one of his friends and another girl and I was a few feet away talking to someone else. I glance over at my boyfriend and I think I saw him rubbing the top of that girls back. He wasn't talking to her, he was talking and looking at the guy.

I didn't pay any attention to it at the time and I never said anything about it to him.

But if he was doing that, what does that mean?
Help! What does it mean if my boyfriend was rubbing another girls back?
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