Holy shit this thread is ridiculous. You flirted with another guy when you had a boyfriend. Oh fuck! Do you tell him when you go to the bathroom, too? If he gets angry about that, he's overprotective. Anything more than that, and you'd be obligated to tell him, but... c'mon, if you couldn't flirt when you had a boyfriend/girlfriend, every conversation with the opposite sex would go:
"Hi. My name's Andrew."
"Hi. I'm Melanie."
"Where are you from?"
"Sorry, I should probably go, see you later."
It's so damn easy to slip into flirting, it's sorta fun and definitely not that big of a deal... if you must tell him, just say "Hey, I didn't tell you this sooner because I was afraid you'd be angry, but I flirted a little with this kid at my church camp. It's no big deal, nothing happened, but I thought it'd be important to let you know." Voila. You've made yourself trustworthy, open, and honest in his eyes, and you haven't really done anything wrong. Your welcome.