What Causes Pain Between The Buttocks?

For four months now I ve had pain in the crack of my butt..doc first thought it was fungi...then I got infection in my foot and it swelled up they took blood and got me on a pill that got rid of that but still have the but pain ...if I put Mupirocin on it then most of the pain is gone but it doesn t heel..went to dermatologist who did a scrapeing of it and gave me pills for seven days only...during that time I still had minor pain but when I finished I had no pain for four days...now it s back and I m trying to figure out exactly what I have as I ve looked on internet as dermatologist said it s e coli infection but everything I ve read doesn t apply to me...so am still not sure exactly what is wrong with me...any suggestions? I see dermatologist again tomorrow