Crush on coworker

So I have a boyfriend that I've been with almost a year. He's really wonderful and I love him. For the past few months, I've had a crush on a coworker. It started as nothing extreme and it's gotten to the point where I think about him when I'm with my boyfriend. The coworker has made slight advances at me (telling me how good my body looks when I was talking to him about dieting, etc.) I feel awful for how I'm feeling. I flirt with him a lot but have never made advances at him. Ive never mentioned to him that I have a boyfriend.

When my boyfriend and I started dating I was still in the process of losing baby weight (my ex husband left me when I was pregnant) and my self confident was at an all time low.

Is there something missing from my boyfriend and I's relationship that I'm able to be swooned by someone else? Or have I just taken the extra male attention too far? I dont know what to do. I feel so awful.