What Causes Irregular Periods And How To Identify Ovulation Period?

i'm 26, 154cm tall and weighs 46kg. my husband and i are trying to conceive for a while now. after marriage my cycle become shorter. before i got married, it was always 28days. now, 18 / 20 / 24days... irregular. but still my period lasted for 6-7days. why is this happening? and how can i count my fertile week if my cycle is irregular..? do i need suppliments such as EPO to help?