probably your friendly anaerobic bacteria spreading out to seek and destroy.

I'd get after it right away with the salt water and baking soda routine. I always do that if I have a slight tooth pain (since last years dental disaster) and it never amounts to much because of that. I think it makes cavities go away i really do. If you've never done that routine before it'll make your gums hurt for s few days but keep at it, the bacteria in your gums simply make your mouth hurt a bit as they die and then your gums are like pink steel next week. sort of anyway.

massage it in real good, gently, with your tooth brush. I add fresh aloe gel to that, to wet it, but water is ok too. or peroxide (without amalgams).