Oh, it's defiantly not septicemia. Septicemia is where an infection in the blood stream, and it'll usually appear as a red spot on your fish, hense why I'm worried about Nya since it's pretty reddish...
Although now I'm wondering if it's possible for fish to bruise, cause she likes to jump at her reflection i the plastic lid of her tank and bonk her head on it :/
Anyway, just check betta talk's list of betta diseases
. They have pretty good general info on betta fish diseases... although if he's behaving normally, aka eating, swimming around normally and not lethargic or hiding... chances are he's fine :)