What Causes Irregular Menstrual Bleeding With Back Pain?

I have been having irregular bleeding for 5 days now, it varies between dark brown and slightly red and isn't heavy but I also have noticeable backache, my period is due on 14/01/2011 and my cycles are 26 days long so I estimate I ovulated on 30/12/2010. I am trying to conceive but also have endometriosis, I went to my GP who said it was normal, however I have never experienced this before.