What's wrong with me and when should I go to the ER for rectal bleeding, very weak and in pain today?

i would say do some research online for the best doctor you can find and then email him exactlly what you just put here cause some thing is going on and you need to find out what it is. also get a second opioion from another doctor. i would go to the hospital and next time you have a bowel movement i would collect some and rush to the er and tell them whats going on.