Is cheating when....

Is cheating when your BF goes out to lunch with a girl without telling you? And complementing her? Then, I found out and we broke up for a couple of weeks but he changed completely 360 it's been a year now. And he's not like that at all anymore...we were rocky when that happend and he swore that he just went to lunch and worked on a project together. Sometimes I feel like I wish that never happend because we have such an amazing relationship now. And it's sad to know that the girl had a boyfriend too. She didn't know that my boyfriend had a girlfriend. My boyfriend is so different now he's like the best but it sucks that something had to happen for him to change completely...any thoughts? Is it cheating? I trust him now...but sometimes it just crosses my mind and think that I can share with you ladies