Hello Doctor. I Want To Consult About My Delayed Periods,

Hello Doctor. I want to consult about my delayed periods, my periods are delayed by 11 to 12 days. I am sure that I am not pregnant. Please help me by letting me know the cause for this delay. I am 33 years old and have not yet planned the family though. I am feeling very slight pain in my breast but don't know the reason behind this. Though i usually feel the same stiffness and pain in breast before my period dates. I am a migraine patient and had sever pain twice last month in Feb and was both the time had gone through heavy pain killer injections. can u please help me and tell me about the cause of this delay, however every time i have my periods on almost right date. Thanks in Advance...