How do I apologize when I don't know what I did?

I recently lost a friend. The thing is, I’m not too sure why. She’s a co-worker and we were friendly and had chats on random subjects on occasion and would sit and talk at lunch. But starting on Monday she seemed to start to avoid me, and today I tried talking with her and she ignored me. She seemed very mad at me specifically while remaining amicable towards others. She even removed me as a friend from facebook.

I have some theories as to what may be the problem but am very unsure of each.
First She has a boyfriend in the army, and although I have bluntly stated I in no way intend to challenge or upset that, I have also noted I thought of her as attractive. I doubt it because she seemed to take it as a simple compliment(my intent) brush it off comfortably and move on to the next subject.
Second I have a tendency to rush out when work is over because traffic is pretty bad in the afternoon around my area; a ten minute drive in the morning becomes thirty to forty-five minutes around the time we get out. I think that maybe she felt I cut off a conversation with her in favor of rushing out. If I did it was completely unintentional. I doubt this one because I’ve been careful to finish conversations before leaving, and doing my best to remember to say “bye” or wave as I go.
Lastly I fear I may have bugged her too many times and she grew tired of me. I sometimes have difficulty knowing if a person is in the mood to talk. this one I actually don’t have much of a reason to doubt beyond she seemed consistently friendly till now.

I don’t necessarily want to be friends again (nice as it would be) Instead I’d be content with knowing what I did and properly apologizing for it.

Any suggestions?