Can a cavity form under the crown of a tooth that had root canal treatment?

A couple of years ago I had root canal treatment, and a crown was placed on the tooth soon after. Recently, I have been feeling a slight pain in that same tooth. I noticed that there is a tiny bit showing from under the crown, and I am not sure if it is my actual tooth or what, but the colour seems to be black. Is that a cavity? and if not, is it possible for there to be a cavity formed under the crown, which is what is causing pain, even though I previously had root canal treatment?