I Slipped On The Ice And Fell On My Back

I slipped on the ice and fell on my back and butt. I first felt it on my lower back but after about 3 hours my back was fine but my butt cheeks are very sore. I decided not to get x-rayed. Last night I think my bowels were beginning to move and I had a lot of pain in my rectum. This morning I had a bowel movement and had a very little showing of fresh blood. I think everything is ok right now. I just want to know what symptoms I should watch for. I don't want to go to the doctors but I don't want to be stupid either.