What Causes Itching Rash On Butt Cheeks?

I am 26 weigh 175 and am 5'9..about a month r so ago I noticed I had a rash on the inside of my butt cheeks that became itchy at times. Later I noticed that my anus became pretty itchy n irritated as well. Blood would appear on my toilet paper and bowl movements have become kinda painful and uncomfortable. Now iv recently noticed that after periods of time my anus has a almost pus like substance that can have a bad smell.. What is all this and how can I make it stop??....I have no medical history