He found something in my old notes..? OMGGGGG!!!!! WTF.....?

long story short. we been on and off for six yrs when we finally get back on good terms and decide to take it slow and possibly get back together something always happens. he went thru my notebook and found some old papers or whatever of two guys i were with.(yrs ago) he flipped because he seen both of the guys at my house before because they are friends with my brother and they have been cordial you know? so hes mad as $hit at me. but what was i suppose to do wait around for him when he did what the hell he wanted to do with whoever? i dont think so. i know hes probably hurt. he made me feel kinda slutty but its not even that serious i apologized but i was kinda upset because i know we could have something good we are just soo young (21) has anyone been in a similar situation? i dont know. some good advice would be great thanks. what should i say to him?