I can understand why you'd feel that way. Your biological parents are part of who you are, and to learn that you have these people that you are related to, in a way, but that you didn't know anything about ... wow.
Aren't there services designed specifically for people who have been adopted? To help you deal with the issues, and to help you find your birth parents, if you wish to do so?
Anyway, feel free to talk about how you're feeeling. It must have felt so strange to have discovered that you were adopted, and you are probably feeling hurt that you weren't told the truth in the first place. Perhaps they witheld the information to protect you?
If I were in your position, it'd be on my mind a lot, too.