I have a light coloured lump about half the size of a pea on the gum below one of my back teeth. it has been there for a while. it isn't painful.

Lumps: The absence of pain does not always indicate the absence of a problem. Many chronic infections lie dormant until one day....Kaboom: severe pain and swelling. That is not not say that that's what you have. See a dentist to evaluate what you see and feel (and what you cannot see and feel).

Fistula "blow hole": I agree with the others. It could be any thing. But in my experience you may have a fistula. A fistula is a volcanic eruption like result of "pus" pushing its way out from an infected tooth. Sometimes a small opening will appear at its tip, letting pus out, acting as as "blow hole" safety valve. With pus not building up significant pressure under the tooth there is little pain. See your dds.

Simple to serious: Could be as simple as a small benign bone growth (exostosis), or as complex as a badly infected tooth (abscess), or one of numerous other problems. Can't do this over the computer, you will need to be examined by your dentist. Take advantage of his/her extensive training and many years of experience.

Vague: Hard to say without an exam and x-ray. Get to a dentist and have the area checked. May be nothing serious or could be an infection. Don't wait.

That could be : A chronically abscessed tooth that opened and been draining out through that plumb in the gums, that's why it's not painful since there is no pressure, you need to have it examined and treated as if it is left can cause more serious problems you don't need to have..