What Causes Pain Before Menstrual Cycle?

Hi doc. I just had my rectum and anus removed and i now have a permanent colostomy bag. I usually get my period on the 20th but last month it all changed to the 26th. my periods usually make me feel pretty bad, back pain, tired, i become more sensitive. oct 1st i will go see a gynecologists but from the hospital i'm taking hydromorphine for pain as needed. i'm usually a tylenol only person but its not helping. what's the best thing i can do today to help the pain? i feel if the period doesn't come in a few more days and my back is still hurting and my tummy feels crampy then it could possibly be something else? i just want to know if this is common or not after surgery? my surgery was the 29th of last month