Rectal bleeding, heavy bright red blood in stool and lower back pain. what should i do?

You need: To see a GI doctor or colorectal surgeon, sooner rather than later before it becomes a life threatening emergency! this does not seem like something you should wait on for a few days to "see if it gets better". If necessary go to the nearest emergency room! take this situation very seriously.

Emergency room visit: It is advisable to go to the emergency room if you feel you are having such large amounts of bright red blood per rectum, especially with associated symptoms - this could indicate a serious problem or it may just be a bleeding hemorrhoid - regardless, you should have a physician evaluate you sooner rather than later, especially if you develop low blood pressure and/or light headedness.