I don’t get it????

So, this morning I wake up to a notification from Instagram and it’s my boyfriends ex and she had liked a photo that I posted for his recent birthday. She didn’t unlike it so I know that she intentionally did it. But what do I do? I keep thinking of reasons she might have done that but I’m not sure. She had cheated on him which is another reason why I don’t get it?

Any advice or stories on something similar, we’ve never met in person she dated my boyfriend in 2015.. I don’t know what to think or do about it.. like do I confront her orrr just ignore it? My boyfriend doesn’t talk to her cause he doesn’t really appreciate what she did to him.