I have a serious question!! i need ur help?

ok so i have a g/f who i am in love with and she has did stuff behind my back before (like in the beginning) but its done and over with and now i jus found out that she is possibly doing it again wit some other kid...ok this is what happend i went out to her car to get something out of her school bag and i find this date book that has like all things that she has done on a calender (like people she hung out with) and she had this one kid written down alot liike more than me and she neva told me bout hangin out wit this kid sooo i ask her bout it and she flipped out sayin why u goin through my stuff so i said i jus stumbled upon it and wanted to know what it was cuz thats the truth and she wouldnt tell me if she hung out with the kid or not she jus kept changin the subject and yelling at me for goin through her stuff and not trusting her and then she broke up with me and told me to have a nice life..Do u think she was cheating on me?