Who's wrong?

I'm under the opinion my boyfriend is being extremely irrational. He seems to think i was flirting with another guy (who happens to be my friends ex boyfriend) due to the fact he messaged me on facebook saying 'hey, sorry for adding you, you just look really familiar'. I replied and said 'no worries, yeah it's because i used to go to the same gym as you so that's probably where you've seen me'. This turned into an innocent conversation between two people who enjoy health and fitness talking about what gyms are good and my friend (his ex girlfriend). I'm a friendly person and i don't like to ignore people, there was no flirting or malice in the conversation what so ever. My boyfriend saw this this morning, called me a slut and lost his shit saying i was flirting and trying to fuck this guy. Forgive me if i'm wrong, but there was no flirtacious intent behind our conversation at all, and it was literally an exchange of maybe 10 messages max. Is he over reacting? Or am i wrong and do i need to apologise?