Why do I get so sick on my period?

Hi all, I'm 15 years old and this is my 3rd year on my period. Somehow, I get very nauseous on my period, horrible abdominal cramping and diarrhea (Sorry If I grossed you out). The pain is so severe, that I can barely eat, and I have anemia because of that. I also suffer from PCOS, and I'm a little overweight, but not that bad. What can I do to relieve these symptoms?