I dont know why i'm always ill!?

I am a 14 year old girl and i am constantly ill! I have had swine flu 4 times and i go home for at least a week every term at school. I have frequent occurances of mastoiditis in my ears and i have had appendicitis. I can;t remember feeling 100% and strong for such a long time and i just permanently feel ill and really week. please help. i just want to feel healthy again.