Help diagnose betta

[AmyLynn]

Last night I noticed Naughty was more lethargic than usual. Just hanging in his plants and not coming up to watch me as much. When he did i noticed that right at the front of his body, underneath where his gills come together he had a huge sore with what looked like a piece of rice coming right out of it. Today he's more active and the "rice" piece is gone, but he has large red sore. Need to diagnose what it is and how I need to treat.

Thanks

http://www.aquaticcommunity.com/fishpictures/data/951/medium/P1030015.JPG

http://www.aquaticcommunity.com/fishpictures/data/951/medium/P1030013.JPG