Attracted to A Coworker?

My boyfriend and I have been together for a little over a year, and we've worked together on and off for the past two years. We've lived together for two years as well. Lately I don't feel attractive to him. Which is making me feel not attracted TO him. I've been fantasizing and flirting with other guys. Especially one coworker in particular. I don't think I could ever act on these feelings, but they're very strong towards this coworker. Even mundane, platonic things he does gives me butterflies. What should I do?