My Dirty Little Secret

I'm (23) in a fully committed long term relationship with my boyfriend (24) of 4 years. Everything is going great so far. I graduated last year and work full time. He's still a student in college working towards his bachelor's. We just moved into our first apartment together in June. Moral of the story is, we got a good thing going here!! YET. THE LUST BUG KEEPS ATTACKING ME. I see these attractive men at work.. I fantasize about them. I find myself talking to guys and not batting an eye when they try to low-key flirt with me. I even regained an attraction to a old flame of mines (never had a relationship but had strong chemistry, sexually and mentally and emotionally). It's just so crazy, I don't know where it's coming from. Me and my boyfriend has been super sexual lately. I've been initiating LOTS of sex. Yet, I still lust!! I don't know what's going on. I need answers. Is this normal??

**UPDATE: I don't want to cheat and I don't flirt back but I also don't rush to say oh I have a boyfriend don't try it. I haven't acted on my lust, I'm just conscious that I'm feeling the lust...