Boyfriends liking his ex’s Instagram posts?

I recently discovered that my boyfriend of over 1 year has, within the last few days, liked multiple posts containing revealing selfies of his ex who, for a while, lived with him and he used to be quite serious with. He does not follow her on Instagram which leads me to believe he looked her up and was seeking her attention for some reason. He has cheated on me a few times before and though I haven’t forgiven him, I have tried my best to move past it and begin trusting him again. I don’t know how to bring this up to and explain that it makes me uncomfortable. I also don’t know if I should actually be upset or if I’m overreacting. Need some advice.

UPDATE:

I confronted him about it and he claims that she never lived with him which is what he told me when we first started dating so don’t know what to believe as far as that. Irrelevant. Apparently she had tried messaging him and he didn’t reply?? But there’s no proof of that so I don’t know what to believe there either. It still doesn’t make sense as to why he’d be liking these selfies and body pics of her.