IDK what it is, never encountered this with my fish or others. Perhaps someone who has had an experience with this symptom will chime in. I guess it could be fluids (swelling), or tissue (growth/tumor; may occur when heavy metals are present in the water), or a strangely-placed fat deposit. I doubt it is a burn (like from a heater) or an infected wound (there would be redness), or an embedded parasite. I doubt it has anything to do with sexual/spawning behavior.
If it was my fish and no one could tell me what it was, I'd make sure my water is perfect (including the absence of heavy metals, including iron) and wait and see, especially if the fish feeds and behaves normally.