Am I crazy for not understanding my girlfriends anger in me friending her best friend on face book?

So I'm sitting in Panera with my girlfriend of almost two years whom I love very much(no sarcasm here). She's on her computer. And asks me, "when was the last time you were on face book?" I say "I haven't been on for a while." She says, "Look now." OK so I pick up my iPhone and go to my Face book app and look at my wall. There is a funny picture there, but since it's on my iPhone I can't see it. So I tap it to make it bigger, but instead it goes to her wall, and I see something there from her best friend who lives on the other side of the country. For whatever reason, I tapped her to see her profile. (I guess I just wanted to look at her profile why is that wrong?) So when I did it said we weren't friends and ask if I wanted to add her. So I said out loud, "Me and _____ aren't friends I'm gonna add her,” and I think I tapped her name. I honestly don't know if I did because my girl immediately said, "NOOO!! don't do that!!!"(not screaming just very stern and deliberate.) I said, "I think I already did." (I really don't know if I did or not) So he says, “I can't believe you did that.” At this point I'm like so confused I had no idea that this was such a big deal. I add people as my friends all the time and could care less who she friends so why would I think twice about friending her best friend someone she talks to everyday? So, she accuses me of doing it as I told her I was doing it so she couldn't tell me no. She also accused me of not caring about her and how she feels. I would do anything for her and do as much as I humanly can for her on a daily basis to keep her happy. She is a great girl and I love her so much. But every once in a blue moon something like this happens that completely catches me off guard and I don't know what to do. I'm a pretty laid back guy especially when it comes to personal issues. Nothing really bothers me so some times it's hard for me to understand when other people are bothered by things let alone pissed, and I really think it can be a problem sometimes because nothing really does bother me. The only things that get me flustered are non-logical situations. So I got pretty riled up over this because I could see no logical explanation for her anger, and she won't explain it she just says that she is. So I ask her a bunch of questions trying to understand where she is coming from, and she gives the same illogical answers like “me and her just don't talk about guys,” “I don't want her all up in my business,” “She doesn't let her guy friends friend me.” (This one really made no sense to me.) All the questions only make her madder which in turn frustrates me more and by this point we are in the car already and we are almost yelling at each other. In my opinion nothing is that important to be yelling about. All of this over an innocent click of a button. I mean really if I thought I was doing something wrong would I have told you as I did it. When I said that she said, “That's another problem you never think before you do things. What!!!!! I know you don't know me but anyone how knows me (including her) knows I think way too much about everything. It actually bugs me how much I analyze things. If I would have thought for a million years I would never have thought she would be mad at me about that. Also, this is a tangent but whenever we do have a problem (which is rare.) she doesn't' let me talk to my friends about It, or anyone for that matter. She doesn't like me saying anything but good things about her to other people. So, here I am. I'm sure she would be upset about this even though it's anonymous. I just don't know what to do can someone help me. Please ask if you need any clarification.