Boyfriend plays down our relationship to his attractive co worker

I realised my boyfriend had left his work bag at home so I went to give it to him, he didn't know I was coming. The reception desk showed me the way up but then a couldn't get the automatic door to his office open. He didnt see me and then started talking about his big night out he was having in Poland. His co worker asked if his girlfriend minded to which he said 'it doesn't matter if she does, she has an understanding that my friends come first as I've known them longer than Ive know her. Girlfriends come and go while friends are forever'.
The co worker smiled and That's when I walked in. They both looked nervous.

I've said nothing since but I honestly think this is him trying to get with his co worker. It's like he's letting her know I'm not 'the one'.
Or am I reading too much into that? My uncle works within the same building and says they flirt quite a bit but he always mentions me throughout the day. Recently he has been mentioning me only when this co worker isn't there and only if she asks about me. Just last week he had told her about the time we split up although he did mention it was only for a week but again I feel like he's trying to tell her all is not rosy. She drunk text him and while he declined coming out with her he was so excited by the text, has told everyone about it etc.

ITa hard for me to discuss these things as he will know my uncle has repeated his conversations to me and that could cause trouble for my uncle. My uncle couldn't have made these things up, he wouldn't have known them to be true unless they were as I don't discuss anything about my relationship to
Him or any family.
Is all this normal behaviour for the office? I don't work in an office so I'm not sure.

My boyfriend used to work in a larger office but is now always where this co worker is in the side office. He seems to want to be near her where possible but does not ever go for lunch with her or leave the office with her.