Am I overthinking this?

My boyfriend doesn’t follow a lot of people on social media, about 100 people on instagram. One of them is a girl that likes every single one of his posts, and also all of his stories. I asked him who she was since she is so active on his social media, but i’ve never heard about her. He says they went to school together, but according to her facebook she went to another school than he did far from where he lived. We made a promise to not have any contact with people we’ve been with or been interested in, and he unfollowed every girl he’s been with on different platforms, and she was one of the girls he unfollowed there. They are only friends on instagram where she likes everything he does. My boyfriend is almost 25, is it weird that he only “keeps in touch” with this girl even if they haven’t talked in years? I haven’t seen any texts between them or any other communication, but it just feels weird to me. It feels like she is hitting on him and that they may have known each other on some other way that from school.