I think i'm falling in love with my best friend. What do I do?

To give a background, I was in a serious relationship with a girl for three years and was dumped on the day I proposed her. It was then I decided, that I will never have emotional feeling for anyone or fall in love again because I don't want to go through the pain of breakup. I have been out of relationships for three years now, although I have a lot of one night stands, without any strings attached and I'm extremely happy with it. Now coming to the point, I moved to a new country after my breakup and made friends with a new girl in my workplace, and we are best friends now. We hang out together all the time, have lunch and dinner together and do everything together. Lately I got a dream that she was kissing some guy and woke up and was feeling wierd. Now whenever she talks with some guy even at work, I get extremely jealous. I think I'm falling for her since I'm hanging out with her a lot. Now I'm having thoughts of asking her out. This is exactly what I'm trying to keep myself away from, emotional attachment with anyone. Despite that I'm having feelings for her. What do I do now?