Help! Sick betta fish?

[Colleen (5'4", 25y)]

My betta seems to have some sort of growth or injury on the side of his mouth. It's red and swollen, so that he can't close his mouth properly. He's also stopped eating. I have cleaned his tank and ordered Ampicillin to give him, but I still don't know what the problem is. There are no other fish in the tank.

Does anyone know what might be wrong with my fish and/or what I can do to help him?