Why am I feeling so insecure about a girl from my boyfriend's past?

My boyfriend and I have been together almost 2 years and are completely in love with each other. We are planning to get married after we graduate from college and get our careers going. I trust my boyfriend, but sometimes I get insecure about other girls. Well, last night I was at my boyfriend's house spending the night with him and he was looking through a drawer for something and came across some paperwork from this junior police academy thing he went to in high school (like 6 yrs ago). He opened it and started looking through it and it was covered with a girl's signature on just about every page and also little notes to him. I could tell he was studying my reaction, like he was showing me this in order to piss me off because he knows how jealous I get about other girls. I recognized the girl's name as well, and I knew she was this girl he talked about when we first started dating. He had told me about how they "talked" and liked each other but nothing transpired between them. I had made him delete her number out of his phone at the beginning of our relationship because they had fooled around (not , but making out and touching). It made me very uncomfortable for him to have her number in his phone, especially since I deleted every guy out of my phone that he disapproved of. So out of curiousity, I looked her up on facebook and she ends up being this total knock-out and to top it off, she is his ethnicity (My boyfriend is Mexican and I am white). For some reason I feel insecure about other hispanic women, like I worry he likes them better but he insists I am what he wants. Why am I so insecure??