Should I be worried? My boyfriend was texting another girl.

So the other night, I went snooping through my boyfriends phone. I know it's wrong but I did it. I happen to stumble across a text convo where he was inviting a girl to lunch. Now I'm trying to keep in mind they've been friends for a good while and he may just truly see her as a friend he'd enjoy hanging out with and I'm fine with that. The convo was started by her asking him a question and he responded. what's up, buttercup? The whole buttercup thing tripped me out... like okay? What is that!? Then he tells her you owe me lunch and she responds with the same. He tells her he's going out of town for something and that she should come with. She said she has work so it's a no-go but that someday it'll work out. I had really upset him previous to this, which doesn't make an excuse, but I'm trying to figure out why he would want to hang out with her... And earlier he had asked me to go with him for his trip. Wow! Guess I'm second option then? I'm really bothered by it. Guys- does this sound like he likes her as more than a friend? I really want to say something, but obviously I can't because he'll know I went through his phone. How can I go about bringing it up?

No, not an overnight trip. Just a day trip for few hours. We had a disagreement that I knew upset him because of something I had said. But we weren't in a fight. Were still together, in fact we've been doing very well. But last night while he was asleep I looked at his phone. But I'm just so confused at the fact he'd call her a pet name like that, then ask her if she was free to hang out this week and suggest her going out with him.