What Causes Abdominal Pain With Nausea And Headaches Before Periods?

hi. i am 19 years old and about two yaers ago i went to my dr because i was having pains in my lower abdomen,he told me it was ovulation and put me on the pill. it is two weeks before my expected period and i am getting these pains, accompained with nausea, headaches and bad gas. what should i worry about? and what can i do?