Hi, looking at the history what you gave ,there can be few reasons to this problem
1) The margins of the crown might not be flushing well with the tooth
2) There can be a possibility of a gap between the to crowns or between crown and tooth due to improper seating of crown , which is giving rise to bad smell and swelling from the gums.
I wouldn't think of a failing root canal at first because symptoms of that would be little different like swelling more towards the root of the tooth and not towards the crown
But yes if the margins of the crown are not seating well on the tooth and there is a leakage , that might cause a failed root canal in future
Next Steps
get a Radiograph done to check the quality of root canal done and to check the seating of the crown
Health Tips
eliminate any sort of vertical crack in the tooth through a dental examination