What Causes Stomach Ache, Nausea And Diarrhea During Menstruation?

Hi, I first had my periods when I was 12.I used to have absolutely no stomach ache until i was 17.Ever since then I have had an unbearable stomach ache on the 1st day of my periods.I went for a scan when I was 21 and the results were normal except for very small cysts which the doctor said was negligible.I still continue to have a terrible pain and tablets provide no relief for 3 hours.Over the last year,I have had nausea and diarrhoea along with the pain.I have even had to delay going to work.Please tell me what could be the problem and suggest me a remedy for the same.