What Causes Burning Sensation In Anal Region?

Hi, I have severe burning sensation in and out of anus area. This is good on since 5 months now and I am not seeing any results. I have undergone lots test but the only thing found was constipation. I have no fissure, piles or any other condition as such but this burning does not go away. There is some stiffness around anus muscles and I am advised to use Diltiact gel and tables along with peg move. I also developed some skin conditions like rashes and itching in the whole body after this problem started. Please suggest.