Strangest reason my 7 year old son has become upset

I am 26/single and have a 7 year old son. While we were visiting my father/his grandfather, my son was in my old bedroom and found one of my old high school photo albums. There was nothing bad or inappropriate in the pictures, just typical photos teenagers take with their girl/guy friends and classmates, proms, football games, etc. After seeing them he became so upset to the point where he was crying. I looked through each one and can not come up with a reason for his anger. ive tried and tried to ask him why. he will not explain, he simply says he hates them, they make him angry, and he wants to tear them up. That was about a week ago, and Im concerned about it beacuse now just out of nowhere he will all of a sudden say, "Mom, Im about to get mad, cause those pictures are in my head again." Its not funny but I almost cant help but laugh because I am so baffled as to what is the problem?! Any help??