Smell coming from gum at front crown

1keith1

Hi everyone back in 2010 most of my front tooth got knocked out during a football game, so i got a crown put on in its place. In the last while (maybe a year or so) ive been having bad breath and i couldnt realise why this was, until about a month or so when i rubbed my gum with my finger above my crown and got an awful smell from my finger (smelling like infection or even feeces) my tooth/crown isnt painful whatsoever and doesn't seem loose at all so im just not sure what the problem could be. Any help or advise would be greatly appreciated,
Here is a picture of the back of the crown, im not sure if this is what the back of a crown is meant to look like or not but i hope this picture might help diagnose my problem..
Thank you very much in advance
Keith