Is it normal that i can't get over it?

I have been with my boyfriend for 6 in a half years.For a year I was working supporting him financially and mentally while he was doing an apprenticeship as a tattoo artist.Well now he is working his dream job.Okay here is the problem.I went to the tattoo shop because my boyfriend wanted me to take pictures of him tattooing his friend.I asked my boyfriend if I could get the camera out of his work locker.I opened it and found tapped on the locker wall inside a nude pic of a female customer of his posing.I got so furious because he never told me or asked me if it was okay.He said it was in exchange for a tattoo she didn't have enough money for and he needed a model for a painting he wanted to paint.I asked why he didn't tell me.He said he knew I would say no and wanted to do something for himself and felt lately I haven't been emotionally there for him since my brother was in the hospital.I asked him why he had the pic tapped up.He said so he wouldn't lose it.I asked him if anything happened between them.He said nothing happened.I asked when this happened.He said after hours when the shop was closed.Nothing seems the same anymore.I feel my woman hood has been put down.I even said I would model for him.He expects me to get over it and I can't seem to.I feel like this is a slap in the face after I helped him.I have never done anything to him like this.Never cheated or kept things from him.I don't know if I should leave him because of this.Please help.I don't want to be a fool.He got mad at me for asking her her side of the story.I think it shouldn't matter if nothing happened.She said nothing did.And she said she was sorry for what happened.She didn't know how I felt about it.