Boyfriend Trouble

I have a gut feeling my boyfriend is doing stuff behind my back. He has this girl he works with she has a boyfriend. She added him on Facebook he said he had to have her on there so things wouldn't be awkward at work. She always messages him and says guess what I found here or here... he says he has no feelings for her and that there's nothing going on. He comes home from work late every day. He got mad at me when I told him I was deleting her off my Facebook he said that I was gonna be gone if I did. He always sticks up for her. He looks and her posts and comment on them and likes them but he leaves mine on unread. Should I be worried? Am I overthinking?