I hope that by the time you get this answer, that you would have already taken your Chi to the vet. It could be his Anal Glands are Impacted and Infected and if they are, the vet will empty them and probably prescribe some Antibiotics to get rid of the Infection. It could be an abscess. I have a Chihuahua named Lucy, who about 1 month ago developed a lump, that was very painful on her Anus. I took her to the vet immediately after I saw it. They lanced it and gave her Antibiotics and Pain Medication and after about 10 days, it was totally gone and hasn't come back. Please, if you haven't already, take him in. If it is Infected, you need to get rid of it, because if you allow it to stay this way, the Infection will eventually get so bad, that's when you are going to really see some problems come up. Lucy is the one in the picture I uploaded!