Me & Gf = 17. I touched her breast above her bra. She was oky with it. Two days later she is angry. I feel like i betrayed her trust. I need to win it back. Please help? 

You just need to talk to her about it. Talk about what her boundaries are and stay within them.

You actually seem very concerned so I wanted to answer and put in my two cents. Even though she is your girlfriend you do NOT know what is in her past. You have no right to touch her with out her permission. For all you know what you say she is "okay with" how do you know she wasn't frozen with fear!?! You need to sincerely apologize to her when she is not angry and keep your hands to yourself. I bet you would rather her be "mad" cuz she had to make the first move rather then you feel guilty and like how you do now because you took advantage of her.

Sometimes a good song will help to understand women.

THIS .. Contrary to popular belief,  has liitle to do with trying to understand "women" .. THIS is about understanding boundaries for HER  .. And no one else. As a couple it is your responsibility to talk and comunicate just where you are prepared to set these boundaries.

Her being "ok with it" (at least at the time) may have been that uncomfortable moment when she really didn't know how to react .. Which is quite understandable given your your young ages and level of experience. Appologize and admit you misread her signals .. And move on. 

In word problems, & (and) means + (add).

i hope you did not mean to suggest that your combined ages is 17.

But seriously, that reaction is not all that unusual.  Humans have a hard time teaching their offspring about sexuality. 

Unfortunately, while we don't want out kids to engage in wanton sexual contact, we frequently wind up shaming our children about sex as a way to discourage their sexual engagement at puberty.

Just make sure she knows that you won't do that again since you now know that doing that (or similar things) is not something she wants you to do.