Blood in stools changed to large white cotton looking things

For about the last 2 months I have been seeing a lot of blood in my stools when I go to the bathroom. I previously have had a tear in the lining of my anus, and there was blood, but it was only on the outside of the stool (and only on some, not all of the pieces). This time, there seems to be blood on it, in it, and on every piece. There was also some blood when I would wipe. Then, all of a sudden it changed and there was no more blood, but there were white things in the stool. It almost looked like cotton or something, and was also in and throughout the stool as well as on it. In the past few weeks the white stuff has gotten more and more, and the last time I had a BM [one of] the stools was entirely covered with this white stuff; it almost looked like a baby that was just born. I have also been constipated for months, and had extreme pain during bowl movements. It sometimes takes up to 45 minutes just to have a BM, and during it feels like something is ripping through. My doctor said that he thinks it is a tear, but cannot explain the white things. I am on narcotic pain killers, and have been for years, and this situation has never happened, and my medication has not changed. I am scheduled for a endoscopy/colonoscopy on Monday, but am just getting very worried and want to know if anyone has experienced this or knows about it.