Well, first, I think you need to be real with yourself in terms of your desires for your marriage...do you want to still be married to your man, even with your problems? Because these feelings may be a sign that your marriage is not working for you and you need to leave.
However, in spite of me saying this, my hunch, based on what you wrote, is that you very much want to be married, and you have no intention on giving up on your marriage or getting a divorce. In that case, I think that you are having a crush on this co-worker. I think that this is normal - after all, you are married, but who doesn't like working with a nice/cute guy?! I would suggest that you take it all in stride and not worry about these being "real" feelings, especially if you know that you would never act on your feelings out of respect for your marriage. Very good chance that these feelings are really a fantasy, and, more importantly, a sign that your marriage is probably in more trouble that you think. So I would recommend that you and your husband seek counseling, so that you can gather more intimacy from your marriage, and you can grow to be more immune to the attention of a nice/cute guy in the future.
It's kind of odd, I had a sort of similar experience. Out of nowhere my ex-boyfriend from over 10 years ago contacted me a few months ago. We had had a good relationship and maintained a friendship for several years after our breakup, so at first I thought nothing of emailing him or calling him thinking he was no different from a friend. Then, suddenly all these feelings immerged - I started thinking obsessively about our relationship, playing lots of "what if" scenarios in my head and such. I was really bothered by these thoughts and feelings because I knew that I did not want to be with him and I wanted to be with my husband (even though our marriage was in a terrible state at the time). So, I decided to tell my husband about my feelings regarding my ex. My husband knew that my ex had contacted me; I made sure that harbored no secrets with him. Looking back, telling him about my feelings was a pretty bold move - not sure if I would do that again! It left him very upset at me and things were weird for a while. But, interestingly, after some time, he actually started wanted to be closer to me. He started talking to me more, he was more intimate with me. I don't really know exactly what went through his head but I suspected there was almost a jealousy issue - like, here is this other guy causing my wife to feel all these things, things she should be feeling about me. I don't know, he just started to come around and in the end, this whole thing actually strengthened our relationship. At the same time, as I got more attention from my husband, I began to lose interest in this other guy - I thought about him less, wrote and called him less, I stopped thinking about our past and started focusing more on my marriage. Those feelings about that other guy dissipated pretty fast, and I felt a lot more secure in my marriage.