What Causes Blood In Stool With Low Back Pain?

I had blood in my stool last month 2 days after my period which lasted 5 days. I went to the ER and they said it is not hemmroids and they thought it could be a small tear in my intestine. I have my period now and am experiencing blood in my stool again. I have been diagnosed with migraines, fibromyalgia and IBS but my cramps and low back pain during my period is getting much worse. I am also exhausted.