I’m nervous my boyfriend is cheating?

A few weeks ago, my boyfriend looked up his ex on Facebook while I was home and hid it from me. I went on his phone to get pictures and I seen that today he looked up a girl from his job and added her on Facebook. She’s young, and single, and attractive, and I know there is no reason why he would need to talk to her about work when his mother works there too and he knows other people there too, so if he needed to know something about work he has other people to talk to. He gets mad if I follow a guy or add a guy on Facebook but yet he just added a girl from work??? That seems weird to me!!! I have a bad feeling and whenever I have a bad feeling I am ALWAYS right. I’m nervous because he’s there for hours on end and I have no idea what he’s doing at work. If I ask him about it, I know he’s just gonna say “I just added her cause I needed the schedule” or something stupid.