I have never heard of this happening! I've been a dental nurse for 11 years, but I'm not at all experienced with kids under two LOL.

I would be inclined to ring your MCHN line, or something like that. It's probably nothing at all to worry about, but in case it's bothering him, it's best to check it out. It sounds like blood clotting in the area, but it's hard to know without seeing. The fact that it's swollen means that it might be sore.

See if you can get some advice from a professional, and please let us know how you go