What Causes Swollen Lump On Perianal Area?

I had a large swollen lump on my perianal area that was sore before it opened up on its own and started to drain. It only drained blood nothing else. The swelling has gone down a lot but there s a n open hole with a red ring around it but also the skin where the hole is has a pale color to it. It don t hurt, but why is it like this.