Advice on a personal situation?

I really have no where left to turn since my best friend is mad at me and my fiancee sucks at advice concerning friends soo hopefully someone here can help...
Starting yesterday my coworker/best friend has been mad at me but she won't admit it (effectively making it impossible to appologize). I know she's mad because she is speaking curtly with me (even calling me ma'am with this super cold inflection in her voice), yet acting Buddy Buddy with the rest of the office (including a coworker who she notoriously feudes with). We haven't argued or anything... The day before yesterday she was absolutely normal with me! I really have no idea why she's acting this way with me and me alone now... I asked her a few times if she's mad, or if I have done anything to upset or offend her and she claims she isn't mad and I haven't done anything to upset or offend her. Thusfar yhe only other thing that she's said is that she's just tired and frustrated with work... But I know her well enough to feel the tense-ness in her words/actions. It's almost like I can see her guard go up when I walk into a room. In all honesty I have utterly no clue what I did and if I did know, I'd do whatever it takes to make it up to her: I have always been able to admit when I'm wrong. I know that no one can tell me what happened to change her attitude towards me but I was hoping that some one may have an idea of how I could get her to at least talk to me? Please help!

If you're already asked her if she has a problem, and she claims she has no problem with you, I can't imagine what else could be wrong. My husband and I had the same problem last year with a lady at our gym. We were always friendly with her and then she started snubbing both of us. We never did find out what her problem with us was, but later found she had some serious medical issues to deal with. Maybe your co-worker is also experiencing some personal or medical situation.

I would just leave her alone, when she wants to talk to you she will.  I get tired of some people also and I just stop bothering with them.  Just wait and see what happens but just let her be.
 

I had the same thing happen at work several years ago.  Not that we were best friends, but we were friendly.  Then one day out of the blue she started acting strangely, like if you've seen the old movies where the women will pull their skirts out of the way if they want to shun someone?  She wasn't wearing a skirt, but it was equivalent.  I figured it was her loss, and went about my business.  But I'm someone who (thanks to some issues from my past) will not ever beg anyone to talk to me.  I know it's harder when it's a best friend, but you can't make someone tell you what the problem is if they don't want to.  Hopefully, if you let her be for a while, things will settle down and get back to normal.

To be honest I gave up being good friends with people that I work with a long time ago.  Maybe I was just very unlucky but it seemed to create problems.  I know it's tough to make friends with the way our society is now, work is just an easy way to do it.  For me I think it was overexposure.  I would work with, socialize with, and talk on the phone to the same person 7 days a week.