What Causes Abdominal Pain And Cramps During Menstruation?

i never used to have bad periods, but for a few months now i get very weak and can hardly stand my legs feel like jelly, and i feel quite light headed, my stomache aches i am concerned as i have never been like this.... i have noticed they have got worse after i had an operation on my stomache last year for periontitis where the cause was unknown. any advice please.....