Body language/my boyfriend

So my boyfriend just had his uni graduation and this girl A I never heard of posted him on her story (just him and no one else) eith a caption "my favourite grad" (I only saw it cuz he shared it on his story), and like it probably wouldn't bother me but like I said he has never mentioned her in the whole time he was at uni, and now our mutual friends posted dew pictures they took during the night and on one of them my boyfriend has his hand around that girls waist and it made me annoyed or a bit disrespected, I mean I never heard of her in 2 years of him studying and now his suddenly her favourite and posses with his hand like that and they look pretty close on the picture.

On the picture there is another girl B on his other side but he doesn't have his arm around her, he only has it around A and on the picture they look like more a couple, think because he only has his arm around her not the other one.

For a context whenever as I joke I put my hand around his shoulder he always says that's more of a 'mates thing not couple' so seeing him pose like that just doesn't feel right, I'm probably over thinking it but needed to say it cuz it's been bothering me

I've posted about this before, but was wondering if anyone who knows body language could tell me if overthinking it