What I've found here is that jealousy depends on how sure you are of your boyfriend's feelings for you. When I was married I knew my husband loved me and would never stray, it never entered my head to be jealous. Now, with my boyfriend, I am the same as you, jealous of the women at work that he says are pretty etc, I know what women at work can be like, with flirting etc.
I think the more in love with someone you are, the worse it is, and as I say, especially if you feel they love you less than you do them. It stems from insecurity. The answer is to take a deep breath, walk away, think logically about it for a while and put yourself in their shoes, i.e. if a guy at work had banter with you, you wouldnt think any less of your boyfriend. The difficulty is though knowing whether your jealousy is unfounded or whether there is anything happening with this girl at work - you have to trust your guy to be honest with you.