should I be upset?

I just found out my boyfriend had sex with this girl I work with. It was before we were dating but we were 'talking' at the time. Also, she's had sex with about 40 other people, although nothing is wrong with that it was unprotected. My boyfriend didn't even tell me this happened because he was 'ashamed' but, I had to find all of this out on my own. Do I have the right to be mad??