Am I being totally naive?!

I'm in a relationship and have been for 3 years, we have our good times but recently I have just been feeling like something is lacking, can't quite out my finger on it but gut instinct says so.
Anyway a few years back I met this guy, total head over heels love at first site kinda thing we spent a few romantic months together and then he moved abroad for work. I was totally devastated but then found my now boyfriend. Recently, he's moved back to England and although we haven't Spoken to or seen each other he keeps randomly popping up on my social media,liking my pictures ect and I honestly cannot stop thinking about him. I have these crazy dreams that I'll be on my lunch break at work and we will suddenly drop everything and kiss one another. Totally crazy right? 
Part of me wants to message him to meet for coffee to just put my mind at ease to see 1. If he will reply 2. If we do meet is he who I remember him being and we still have this great spark or is he a total idiot. 
I don't want to go behind my boyfriends back and I am certainly not a cheater but I almost feel like I'm emotionally cheating on him already. What should I do? Stop being silly and wake up and see what's in front of me or take the risk and see where it goes? ...