If it is ulcer your resistance is low so some infection occurs. Maybe a small tear there and of course the mouth is always full of bacteria so got infected. It will go away once your health improves.

If it is nanah under your gums you better go to dental surgeon ASAP. My aunt had gum disease where all her teeth kena extracted by age 28. No teeth at all.She had this extreme pain and nanah at the roots of her teeth.Really really painful and gross and smelly. Please go to your dental hygienist NOW.