It could be an ulcer, which is a bacterial infection and it gets worse quickly, however there are plenty of anti-ulcer treatments. The only other thing I can think of is mouth rot, which is like fin rot, but on the mouth, if it is this the fish's mouth will look decayed and the red could be one of the patches of fluffy stuff that can be either pink or white. One of my neon tertras had this and medication after medication all that happened is that it went away for a few days. From the picture though my best guess would be that its and ulcer. If i am right, watch that the fish doesn't get a fungal infection around the ulcer though, as this can lead to further problems.