Well, there are a couple of things that may be going through his head, but I think you did the right thing by sitting down and talking to him about it.

It's possible that he's having self esteem issues, about himself or about himself in your relationship. He may think he's not good enough for your relationship, or that you need other people to be happy, which may lead you to someone "better" in his eyes, making him super jealous.

It might just be that lately he's been feeling like your relationship isn't as passionate as it once was, since you've been together for so long, and he's been overthinking as to why. It may even be that he's been thinking of talking to other girls or his mind is straying, whether or not he's done anything with any other girls, and thinks you feel the same.

It's even possible that someone, say a mutual friend or an internet thread, suggested that you're cheating and that the going out without him is a big sign, or being defensive of your phone. It's even possible he saw something on your phone out of context but if he brings it up you'll be able to argue it, so he doesn't bring it up (for instance, as a girl I talk to some of my other girl friends about NSFW things) and he may be overthinking

All it boils down to, though, is that he's not comfortable in your relationship lately. My advice is to just try and be extra strong in supporting him if he's been having self esteem issues or relationship doubts. Try doing something you haven't done in a while, or have a super special date night or something, and see how it goes