Mostly due to inadequate hygiene (floss, brush), localized infection, "pockets", can be seen (uncommonly) with deficiencies of vitamin c
, folic acid
Can be seen due to some meds, can be seen (uncommon to rare) with some leukemias, lymphomas; allergic reactions to flavorings (cinnamon among them). See your dentist first. Can get special mouth washes, checked for other problems.