I have this weird thing about eating at school?

My mom makes me sandwiches to eat at school, but I never eat them anymore. I haven't always been like this, it started when I was 13, so last year. I weigh something around 85 pounds and I'm about 5'2. I have been the same height for about a year and a half now, and I used to weigh around 100 pounds, and my friends say I have significantly lost weight recently. I'm not anorexic, because I'll go home and eat like normal, and I'll eat the same amount, maybe more, than most people. When my mom found out I hadn't been eating, she was mad and upset with me. I told her it was because of the school hygiene, but I'm not really a germ-o-phobe in any way at all. Why do I feel so uncomfortable eating at school, is it subconscious and is there a way I can stop it?