Whats wrong with my cat?

my cat had a bump on his back towards his tail yesterday. today the bump is gone, but my cat has been cleaning the area and the hair has fallin out. and a redish irrated abd bloody area has appeared. later today it was bleeding and was pusing perfusely. he was an active cat and now he is just laying in one place for several hours. he seems lazy and doesnt have an appatite. what could be wrong with my cat? is he going to die soon? i called the vet and she wont call me back and is rarely in here office. i dont know what to do!