Boy problems!

Okay so I have a boyfriend and we have been together just over a year! We've had problems recently just like any other couple! But one of his mates messaged me recently, not flirting or anything, just a simple "hiya, was nice meeting you" and because I'm just a friendly person we just got chatting, and I can't help it but it feels so right with him, and there's just a spark between us and he said he feels the same! We have actually fallen for each other but not intentionally! I don't know what to do! I've actually never felt like this about anyone!! Advice please?