I need advice :/

Okay so I met up with my ex about a month ago and we ended up having sex, we didn’t speak much after it happened, then two days later I’m scrolling through my Instagram and see that he’s posted a picture of him and his girlfriend!!!!! Just want to make it very clear I had no idea he had a girlfriend or I would’ve never done it so should I tell the girl? I have concrete evidence of it happening

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