What is the pus filled growth on my gum?

[frank76]

I had a root canal done on the last tooth in the back of my mouth about 2 years ago and I've developed a pus filled growth between that tooth and the one next to it. My dentist says it could be an infected root canal. He took an x-ray but he saw no infection on that certain tooth or the one next to it that has a filling. The pus filled growth has subsided quite a bit but the tissue is still there. It has been there for almost 4 weeks. There isn't much pain. It's just a bit sore and I keep the area very clean. Im going to see an endodontist next...Anyone know what it is or why it's there? Is there an infection that cannot be seen by the x-ray?