What Causes Severe Pain During Menstruation?

Hi, may I answer your health queries right now ? Please type your query here...I'm 27yrs old and I have 2children 6 and 8 yrs old,I have bn on the pill since havin my 2nd child..last week I had my period and it was quite painful which was unusual...now ian sufferin from pain again..there is no bleeding but is very painful..why is this happening all of a sudden