What Causes Delayed Periods?

Hi i am 26yrs old & i am sexually active with my partner. I live in remote country and have a poor health system. I have had irregurlar periods before but now i have not had my period since OCT-NOV last yr2010. I have had a blood test & ruled out pregnancy.i have gained weight rapidly over the last yr, i eat well i dont fill stressed i dont do much excerise i like a drink alcohol beverages can someone explain why this might be happening to me?

i fill like i have period pain present but i am also taking a stong antibiotics for a ear infection causing me diarrhea...