Did I take it too far?

This is not a matter of your girlfriend not trusting you or hiding something from you purposely, this is about your girlfriend keeping a promise she made to a very ill friend. As her boyfriend you should understand that there are loyalties we should keep with our friends even when we have a partner. If this would have been something that affected your relationship directly, rest assured she would have said something to you, but since it doesn't involve or affect your relationship, and it was asked of her to keep it a secret, she felt she had to respect her friends wishes and keep it quiet.
The friend that told you did not respect this poor girl's wishes and that is what's wrong here, not your girlfriend being the loyal friend that she is. And if you are thinking of dumping her just because she stayed a loyal friend, then you're being selfish and stupid. Don't say you give her space and do not ask she tells you every little thing when you are getting all riled up over her keeping a friend's secret that never even involved you.

Your overreacting, your friend have cancer might not want anyone apart from the people she told to know about her condition, your girlfriend is just keeping her promise, if she told you about this she broke her own promise and trust. If your mature enough like you said, you would understand why she didn't tell you this, your mixing up friendship promises and trust with your own personal love relationship trust.

For example, If you have a guy friend going out on a late night to a pub having few drinks and hugged this girl, he then ask you to not tell his girlfriend(His girlfriend is best friend with your girlfriend) about it because it was just normal hug and your guy friend had no feelings for that pub girl. Would you tell your girlfriend and break your guy's promise/trust or keep the promise?

Ok kiem-gg. You need to chill. He's asking for advice, not for a lecture from you. Anyways, I can understand why you're upset Anonymous. Obviously you want the one you love to have complete trust in you and to put you above most other people in her life. However, if this is just an isolated incident, I think you can take comfort in the fact that she was probably just honoring her friend's wishes. Who knows, she might have even stood up for you when her friend told her to a keep it a secret. She might have said you'd be able to handle it, but maybe your friend didn't give in. So if otherwise your girlfriend seems to trust you, I would let it go and comfort you friend with cancer as much as you can. If however, this is just one of many other similar trust-issues, it might be time to sit your girlfriend down and tell her, directly, how you feel.

You are turning a serious health matter for a friend a yours into some kind of personal slight. Stop it. It doesn't matter what or why she never told you.