Why would he do that?

My boyfriend of 5 years, we live together and everything!! Well a little back up story, when I met him I wasn’t looking to date no one and neither did he, but we met and we clicked right away, but he had a girlfriend, which the relationship was already broken cause she moved away and he couldn’t tolerate that, and she used to call him but he’d rather just to not talk to her as the year passed, (I believe they only dated for 1 year) anyways! We met we clicked we started dating and then I found out about her, and told him I didn’t want him if he was still with her but he said I’d be rude to end things over the phone and that she was gonna go visit him next week (keep in mind this was 5 years ago) so the day came, he called me told me he was gonna go see her and tell her it was over (which he did) and we continued dating until today, but one day he opened his Facebook (he doesn’t like it that much I guess) and we where looking together and he looked her up, I was like why did you looked for her on Facebook? And he was like that was a long time ago! And then I ask him if I could see the activity log and that was from 1 year ago, meaning he opened his Facebook account (which I thought it was deactivated at that time) looked her up and then closed his Facebook again!!!!!! I don’t know what to think and when I ask him why he just say I don’t know I have no reason to tell you why I did it!!

So I’m over here like 🤨 and thinking he may still have feelings for her 😕 any advice?