TBH I personally can't think of any illneses at all....
I'd go with a physical injury, do you keep it with other angels?
Maybe its a result of lip-locking battles?
Could be a sign of internal bacterial infection, keep this in mind but its seems to be fairly rare as a symptom, are there any other symptoms?
Any deaths recently? Or other illnesses?
Your two best bets are too post a picture if you can and also repeat the post in the new world cichlid subforum (devious linkage); He went that way--><---Nope it's him
In case its something specific to Angels or they have any thoughts on the matter.
Asides from that this is basically a bump.