People call me hypochondriac because I'm off school alot?

Ive been off school this week because I've had a really bad cold and I just received a message from a boy in my class (my ex.. we dot get along) saying "your a hypochondriac, ha ha!" , don't know if I'm being babyish but I hate to think they are all talking about me, I am really insecure when it comes to people talking about me, is this normal? and what the hell is hypochondria ? and what should i say in reply to him?