Big Red Bruise/splotch On Fish's Mouth

[alfonzette]

I think my fish is dying. She just developed this thing on her mouth within the past day and she is now twitching at the bottom corner of the tank. any help? suggestions?

 

[WILDER]

Need more info I'm afraid.
Size of tank in gallons or litres.
How many fish and which type.
Water stats in ammonia,nitrite, nitrate and ph.
Has she banged her mouth on something in the tank.
It its red sounds bacterial.