The messages that your ex girlfriend have been giving you in the past (when you broke up because of her unfaithfulness) and currently, when she cheated on her new boyfriend with you, are quite clear to an objective observer. She had even verbalized her feelings of not wanting to commit, feeling she's too young for a serious relationship, etc.
What you may be feeling is disappointment deep down inside which, is difficult for anyone to deal with disappointment. You are confused because she had engaged in a sexual act with you. Some women being more promiscuous would do so. That does not indicate love or any deeper feelings than the enjoyment of the sexual act. This also applies to men as well and they have been stereotyped all of this time, when in fact, women can have strong libidos and not necessarily want to commit to any oner partner.
A good predicament of future behavior is past behavior. Her behavior had burned you once in the past. Now, it is up to you to decide how you want the future to look like. Relationships take two people who are committed and faithful. The other stuff is dating, fooling around and playing the "I don't know what I want, I have feelings but.."stuff.
You sould more mature than she does.