You're an atheist you can't do that

I am atheist that works in a church, well it's not the Church its affiliated with the church. 
We have two kinds of music we can listen to, country or Christian. I cannot stand country music it's awful to me, I would rather turn on the Christian radio station. I sing to it and hum to it and some songs are actually really good. I work with a girl who was totally friends with me before she found out that I am an atheist. She is like completely disgusted that I am even there because I don't even believe in God. Although my job has nothing to do with god- at all. And I do my job and do it well.  She walked in as I was singing along to a Christian song, it's just one of those songs that I think is absolutely beautiful. She was like "why are you singing to this? You don't even believe in it? That makes no damn sense" 
Um well it's either that or I yeehaw it up in here which makes me want to rip my ears off. 
Religion does not bother me unless it's being shoved down my throat. Even though I am outcasted (and believe me I am outcasted because I'm not only an atheist my views go completely against the people I am surrounded by, I don't fit in here) I go on with my business, I don't have to be your friend to be friendly to you even if you're an asshole to me about my beliefs or in this case lack of and by now I have been there for so long my coworkers get along with me and work with me fine. I have friends outside of work, i have never really worried about the people at work liking me lol but this one girl, I don't even know where she gets off. Why does it bother her so much? It's like she can't comprehend how you can't believe in God. It like infuriates her to the point that she went from asking me to hangout to totally acting like I have the Black Plague.  What do you think about me? An atheist that sings to Christian music and work in a religious setting? I happen to think I am not too bad lol