Is it normal that i have confused feelings?

I have been with my boyfriend for 4 in a half years.We tell eachother everything.There was only twice I got my boyfriend hiding something.Well this time it's pretty big.My boyfriend is a tattoo artist and one day when I was visiting him at work I had to grab his jacket out of his work locker because I was cold and when I looked straight in the locker there was a nude pic of a customer he tattooed.He said it was for art reasons he took the pic to make a painting and needed reference.She also didn't have enough money to pay for her tattoo.I asked why he didn't tell me.He said because he knew I would not be okay with it and wanted to do something to expand himself in art.I asked why he had her pic taped in his locker.He said so he wouldn't lose it.The thing is he had another copy in his locker in a folder.I asked him if anything happened like did the kiss or have sex he said no.He said all she said to him was if you didn't have a girlfriend I would so date you.I asked her about it and she said nothing happened and my boyfriend was respectful.My boyfriend got really mad I asked her.I said what does it matter if nothing happened.He said that she is crazy and doesn't want her to cause problems for him at work if I give her attitude.I talked to her and she hasn't done anything crazy.She told me she is happy dating some new guy she met.I am still with my boyfriend but how do i really know if he cheated on me with her?My feelings keep going back and forth with him.