There's this girl...

So I really need advice. I have been in a healthy straight relationship for 3 amazing years. I am so in love with this guy. But I have recently been in a new job, and there's this girl at work, we flirt all the time and I get butterflies.. I've had feelings like this before but I never thought anything of it. I've never persuaded anything or acted on any of my past feelings as I have been in a relationship since I was 17 so I missed out on the "discovery stage" .. I just don't know what to do. She is also in a serious relationship but makes it clear she's flirting.. I'm just all over the place! I dare not say anything to her because it could only be a one sided thing.. 😩