Swollen lump on dogs anus.(pics inside) What is it?

Since about 4 monthso on the left side of my dog's anus(sorry i dont really know the specific terms) there is a lump or it is swollen. He can poop normally with no problem , and he acts healthy as usual. He is about 8 years old . I took some pictures , can you people help me know what is this?

Note : its pictures of my dog's *** with a lump next to it.

http://imgur.com/a/1Sxg5#0

[Misfits4Me]

Welcome to the forum......Goodness, your dog has had this for 4 Months without a Vet check? Here's a link (From this forum) that will explain what your dealing with:

http://www.medhelp.org/posts/Dogs/Something-going-on-with-my-dogs-butt/show/1794811